题目选项分析

Whereas a ramjet generally cannot achieve high speeds without the initial assistance of a rocket, the ability of scramjets, or supersonic combustion ramjets, to achieve high speeds is because 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.