题目选项分析

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.

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.