Sentence Correction

51. Whereas a ramjet generaly cannot achieve high speeds without the initial assistance of a rocket, high speeds can be attained by scramjets, or supersonic combustion ramjets, in that they reduce airflow compression at the entrance of the engine and letting air pass through at supersonic speeds.

A. high speeds can be attained by scramjets, or supersonic combustion ramjets, in that they reduce
B. that high speeds can be attained by scramjets, or supersonic combustion ramjets, is a ressult of their reducing
C. the ability of scramjets, or supersonic combustion ramjets, to achieve high speed is because they reduce
D. scramjets, or supersonic combustion ramjeets, have the ability of attaining high speeds when reducing
E. scramjets, or supersonic combustion ramjets, can attain high speeds by reducing


































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My answer is D, and O.A is E. The reason I chose D is because that I have concern about the usage of passive voice whenever the word "by" appears in the sentence. In this case,"high speeds can be attained by..." rather than "high speeds by reducing". Why we don't emphasize the form "be+past tense of verb+by" here?

Thanks.