问题补充：
–It’s dangerous here. We must not let people fall down.
--You’re right. Come on!
A. stop... by falling
B. keep...falling
C. protect...falling
D. prevent...from falling
答案：
A. stop... by falling
B. keep...falling
C. protect...falling
(答案→)D. prevent...from falling
解析：划线部分的意思为阻止....做某事故选D
