一个直角梯形，下底40米，如果上底增加38米，就变成了正方形。原来梯形的面积是多少平方米？
（40+40-38）×40÷2=840（平方米）
—What did the teacher say just now?
—Sorry. I didn't catch it. I _____ something else.[ ]
A. think
B. will think
C. was thinking
D. had thought
水轮机转轮下环的作用是什么？
