practice test

Whereas high speeds generally cannot be achieved by ramjets without their initially being assisted by a rocket, a scramjet, or supersonic combustion ramjet, can attain high speeds by reducing airflow compression at the entrance of the engine and letting air pass through at supersonic speeds.

B. Whereas a ramjet generally can not achieve high speeds without the initial assistance of

C. With ramjets, high speeds generally are not able to be achieved without initially being assisted by

Answer: B

其實我找不到主詞吔, 有好心人翻譯一下嗎...和解釋一下句子的structure