题目选项分析

Whereas a ramjet generally cannot achieve high speeds without the initial assistance of 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.

With ramjets, high speeds generally are not able to be achieved without 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.