please help GWD grammar

Q24

Although improved efficiency in converting harvested trees into wood products

may reduce harvest rates, it will stimulate demand by increasing supply and

lowering prices, thereby boosting consumption.

A.     in converting harvested trees into wood products may reduce harvest

 rates, it will stimulate demand by increasing supply and lowering prices,

thereby boosting.

B.     In converting harvested trees into wood products may reduce harvest

rates, demand will be stimulated because of increasing supply and

lowering prices, which boost.

C.     Of converting harvested trees into wood products may reduce harvest

rates, it will stimulate demand by increasing supply and lowering prices,

which boosts.

D.     Of harvested trees being converted into wood products may reduce

harvest rates, it will stimulate demand, because it will increase supply

Answer: B, but I think it should be A because "it " denotes "imroved efficiency".First it reduce

sth, later it  improves sth . Futhermore, even if demand  is correct as a subject, "which boost" is wrong

in grammar, should be " which boosts".

Navigators have known for thousands of years that the ocean

has variable currents, but it is only in the last half century that

a reasonably clear picture has emerged of the patterns and

causes of ocean currents.

A.     a reasonably clear picture has emerged of the patterns

and causes of ocean currents.

B.     a reasonably clear picture of the patterns of ocean

currents and their causes have emerged.

C.     a reasonably clear picture emerged of ocean currents,

their patterns and the causes of them

D.     there have emerged a reasonably clear picture of the

patterns of ocean currents and what caused them.

E.      there had emerged a reasonably clear picture of the

patterns of ocean currents and their causes.

Answer: A  I think "has emerged" can match "in the last half century

answer should be E.

24.it 的指代是有问题的，有指代前面整个从句的嫌疑，在GMAT里，it绝不能指代整个句子，至少这个指代是不清晰的。（有可能指代efficiency也有可能指代从句)

关于主语问题：although引导是让步状语从句，主语为什么不能是demand呢，当然可以是。句意，“虽然harvest rate可能降低 ，需求却增加了”，很合理啊。

which 只能修饰或指代离他最近的哪个词，在这句话里是increasing supply and lowering prices,是复数，当然要用复数谓语boost。

39.首先，关于你认为的时态问题，in/withen the last (past,recent)+时间量是应该用完成时的标志，可以是现在或过去完成时，而不是只能用过去完成时，需要进一步判断，而这句话中，but 表明其前后部分谓语应该是平行的，前半句用的是have known, 现在完成时，后半句也应该用现在完成时。

A选项优于E选项在几个方面：E选项中There had done 结构不对，their 指代不清，可指代patterns或currents.时态不平行。

３９．我选E是因为是否把它作为倒装句看there 指ocean什么的

我仔细想了想，同意２４选B , 楼上说得有道理，但３９我还是认为是个倒装句型