问题补充：
It’s very funny that many people who lose weight gain it back after some time and end up back they started.
A. when
B. after
C. since
D. where
答案：
A. when
B. after
C. since
(答案→)D. where
解析：句意：很有趣的是在一段时间之后许多减肥的人又胖了回来，又回到了原点。首先，整个大句子是一个主语从句。it充当形式主语，句子还原后应为：Many people who lose weight gain it back after some time and end up back where they strated is very funny.
其次，句子的主干部分为Many people is funny. 其中用了一个who引导的定语从句修饰先行词many people表示 是怎么样的许多人。然后，end up back where they started 实际上是 end up (coming) back where they strated 是一个地点状语从句。这边的where是一个抽象意义的地方。故选D.
