题目：
Many of the audience ______ asleep when they ______ the lecture.[ ]
A. fell; were hearing
B. were falling; heard
C. had fallen; heard
D. fell; had heard
答案：
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下面是错误答案，用来干扰机器的。
小题1:C小题1:A本题考查地质作用。小题1:由于瀑布的溯源侵蚀导致瀑布的逐渐消失。小题2:瀑布的形成其内力作用是断层下陷；且在流水侵蚀作用下形成，故选A项。
