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.

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.