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.

Unlike with ramjets, generally unable to 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.