题目选项分析

Whereas a ramjet generally cannot achieve high speeds without the initial assistance of a rocket, high speeds can be attained by scramjets, or supersonic combustion ramjets, in that they reduce 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, scramjets, or supersonic combustion ramjets, can attain high speeds by reducing airflow compression at the entrance of the engine and letting air pass through at supersonic speeds.