跪求两道prep语法题解答！

请教大家一个问题                    
Soaring television costs accounted for more than half the spending in the presidential campaign of 1992, a greater proportion than it was in any previous election.
A.    a greater proportion than it was
B.    a greater proportion than
C.    a greater proportion than they have been
D.    which is greater than was so      
E.    which is greater than it has been
        这一题正确答案为b，我理解是这一题如果选择b不就是比较对象不对称吗？答案说原句补完整是这样的：Soaring television costs accounted for more than half the spending in the presidential campaign of 1992, a greater proportion than（the proportion）（television costs had accounted for of the spending）in any previous election；看完答案我明白了这一题省略了，使得proportion和一个时间短语比较，但是请看另一道题，这道题就明确说明不能名词和时间作比较
Today’s technology allows manufacturers to make small cars more fuel-efficient now than at any time in their production history.
A.    small cars more fuel-efficient now than at any time in their
B.    small cars that are more fuel-efficient than they were at any time in their
C.    small cars that are more fuel-efficient than those at any other time in
D.    more fuel-efficient small cars than those at any other time in their  
E.    more fuel-efficient small cars now than at any time in
这道题C正确，解释是； those合理的指代了cars，使比较对象对等。该题目句子合理的意思应该是，现在造出的车，比汽车生产史上其它任何时候造出来的车要更省油；而不是某一既定的车子在现在比在过去省油，避免这种错误的方法在于，保证句子是名词或代词cars/those之间的比较，而不是表时间的副词或介宾语词组的比较。smallcars that are more fuel-efficient than…这种定语从句结构是没有歧义的。



这样的话这两道题不是矛盾吗？请高手解答！拜谢！

I'm not NN, but if I can be so bold...

第一题，你说“看完答案我明白了这一题省略了，使得proportion和一个时间短语比较”。这是不对的。
Things being compared SHOULD ALWAYS be parallel!
The parallelism consists of two things :
 1. Grammatical parallelism.  For example, you can compare a noun to a noun.
 2. Logical parallelism.  Things being compared should be COMPARABLE! They should be things of same type, in same situation, etc.  Therefore you can never "使得proportion和一个时间短语比较" - that does NOT make logical sense.
Back to this Q,
Soaring television costs accounted for more than half the spending in the presidential campaign of 1992, a greater proportion than it was in any previous election.
A.    a greater proportion than it was
B.    a greater proportion than
C.    a greater proportion than they have been
D.    which is greater than was so      
E.    which is greater than it has been

In the correct choice B, the author is comparing an adverbial phrase (written in the form of a prep phrase "in any previous election") to another adverbial phrase (written in the form of a prep phrase"in the presidential campaign of 1992") . It's fine to omit words as long as there is no ambiguity.
Analogy (Manhattan p. 129):
I walk as fast now as [I walked] when I was younger.

Note that in this sentence, the 2nd adverbial phrase "when I was younger" directly follows "as"; in our sentence above, similarly, the adverbial phrase "in any previous election" directly follows "than". This is how the omission - or so called "Elliptical Construction" - works.

If you asked why choice A is wrong, I would say, the latter part of the sentence "a greater proportion than it was in any previous election" is indeed an Absolute Phrase (a proportion + modifier), NOT a clause (with subject + verb) - there's no verb here!  Therefore, you can not add "was" or any other verb after "than". If you do add "was", you are actually comparing a noun (a proportion) to a clause (it was in ....), and this comparison is not grammatically parallel.
Analogy:
        "an apple bigger than that one"   Correct! You are comparing a noun to a noun (a pronoun has the same role as a noun).
        "an apple bigger than that one is"  Wrong! You are comparing a noun ("an apple") to a clause ("that one is")

Your 2nd Q is a great example illustrating how we should pay a close attention to the nature of the comparison.
Today’s technology allows manufacturers to make small cars more fuel-efficient now than at any time in their production history.
A.    small cars more fuel-efficient now than at any time in their
B.    small cars that are more fuel-efficient than they were at any time in their
C.    small cars that are more fuel-efficient than those at any other time in
D.    more fuel-efficient small cars than those at any other time in their  
E.    more fuel-efficient small cars now than at any time in

What do the phrase "at any time" modify?  Let's leave the question here for a moment.
This sentence wants to say:
Manufacturers can make [small] cars[that are more adj.]thansomething.

Here "[that are more adj.]" acts as an essential noun modifier, which modifies "cars".  
We are comparing cars tosomething, and therefore "something" placed here should be grammatically and logically comparable to "cars".

C is correct;  we are comparing "[ ] cars [ ]" to "those [at any other time in ....]", and here "those" refers to "cars", and "at any other time in ...." modifies "those", i.e. it acts as a noun modifier.

B.    small cars that are more fuel-efficient than they were at any time in their

Here "at any time in their ..." modifies the verb "were", and thus acts as a verb modifier (like an adverb, here an adverbial phrase).
Also note that "they" refers to "small cars".  So, the actual comparison in choice B is between what the "small cars" are now and what those same cars were in the past. This comparison does not fit into the context here.

A.    small cars more fuel-efficient now than at any time in their

A is ambiguous. We can interpretate the meaning as that in either B or C.

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So, the difference here between the above two Qs is, back to what I asked before, "WHAT DOES THE PREP PHRASE ("at ...." or "in ....") MODIFY?"
In 1st Q, the prep phrase modifies a verb (serves as an adverbial phrase); in 2nd Q,  the prep phrase modifies a noun.

To decide whether or not we can omit words, we should 1) figure out the nature of comparison, and then 2) decide whether there is ambiguity if we omit words.


btw:
Here is a good example on Page 135 in Manhattan SC
WRONG: The clothes inside the store looked more appealing than on the racks outside.
Correction (a): The clothes looked more appealing inside the store than on the racks outside.
Correction (b): The clothes inside the store looked more appealing than (did) those on the racks
outside.

See? Similar to what we are discussing here!