平行为什么没有不出介词？？

212.
                The Baldrick Manufacturing Company has for several years followed a policy aimed at decreasing operating costs and improving the efficiency of its distribution system.

(A) aimed at
                decreasing operating costs and improving

(B) aimed at the decreasing of operating costs and to improve

(C) aiming at the decreasing of operating costs and improving

(D) the aim of which is the decreasing of operating costs and improving（A）

(E) with the aim to decrease operating costs and to improve

The best choice, A, offers an adjective phrase (aimed at 是形容词短语，不是动词过去分词) unequivocally modifying policy and exhibiting grammatical parallelism (decreasing... and improving). In choice B, the gerund the decreasing is not grammatically parallel with the infinitive to improve. Likewise, in C and D, the decreasing of... costs is not parallel with improving the efficiency. In E, the infinitives to decrease and to improve, while parallel, are less idiomatic than the prepositional phrase of decreasing... and improving in modifying the noun aim. Also, with the aim... improve can easily be construed as referring to the Baldrick Manufacturing Company and so does not refer unequivocally to policy.

improving前面为什么没有补出介词at 呢？不是说平行的时候介词一定要补出的么？？

好像有的情况下直接就省略了，有些有没有，有具体的规定吗，比如说：

823.The Rorschzch test is gaining new respect as a diagnostic tool because it takes only one hour to expose behavior and thought processes that may be unlikely to emerge in other procedures or weeks of ordinary interviewing.

(A) that may be unlikely to emerge in other procedures or weeks of ordinary interviewing

(B) whose emergence is unlikely in other procedures or weeks of ordinary interviews

(C)that might not emerge in other procedures or in weeks of ordinary interviews

(D) that may not emerge under other procedures or weeks of ordinary interviews（C）

(E) unlikely not to emerge during weeks of ordinary interviewing or in other procedures

大全823题，正确答案就是that might not emerge in other procedures or in weeks of ordinary interviews, in 没有省略，
        可是在别的地方都是省略的啊，
        还有一道prep 破解版的题

The Sports Medicine Programs of the Olympic Training Center, a complex where final tryouts are held for athletes representing the United States in the Olympics, is geared toward enhancing athletes' performance and toward their preparation for international competition.

(A) is geared toward enhancing athletes' performance and toward their preparation

(B) is geared to enhance the performance of athletes and to prepare them

(C) are geared to enhance athletes' performance and their preparation

(D) are geared toward the enhancement of athletes' performance and toward preparing them

(E) are geared toward enhancing the performance of athletes and preparing them

答案是E, 介词省略了，
        虽然E的确是这几个选项里的优选，但是介词是可以可省可不省的吗？

(A) by using the company’s earnings and to profit

(B) by using the companies’ earnings and by profiting

(C) using the companies’ earnings and profiting

(D) with the company’s earnings, profiting（E）

(E) with the companies’ earnings and to profit

答案是E，我觉得在A和E中选了一下还是选了A，但是A也有缺陷就是by+动名词
                和by+名词的平行了。。

那么难道E也准确的吗?就是with +名词和by+名词的平行？？

期盼NN 的解答，