一道选择题

你好。不选C，因为A与C在这里是同一个意思，都是表原因的。如果since或because去掉一个，就对了。这样的话，答案就是when或者since/because中的任何一个。有 if 的话，也是对的。
since 、when、because 或者if 都是对的，条件是：它们中只有一个出现在ABCD的选项里面。

不选C.
when表示假设条件.
since/because表示原因, 本句是条件句,并非是对已经形成的结果阐述原因.
since 一般指自从........以后.

见下例,看是否能帮你理解该问题.
People become uninhibited when they drink.
人一喝酒就会变得肆无忌惮。
People get rude and whiny when they are exhausted.
人们在精疲力竭的时候会变得粗野，爱发牢骚。

She can only do light work since her accident ..
她出事以后只能做轻活儿了。
Mary has pined away since she fell ill.
玛丽自生病后变得很憔悴。

He left his job largely because he was homesick.
他辞职不干主要是因为他想家。
I didn't eat the fish because the smell put me off.
我没吃那条鱼，因为它的气味使我恶心。
He cannot run because he has a weak heart. 他有心脏病，不能跑。

because
1.针对情况解释原因（非推理所得的原因），有明显的因果关系
2.从句不放句首
3.能用于强调句，since as for不能
It is because John is honest and diligent that he is popular with his friends.
正是因为约翰的诚实勤奋使得他受到朋友们的欢迎。

when从句可用终止性动词和延续性动词when的常见句型：was doing …… when did ……
was about to do …… when did ……
hardly when ……
had hardly …… when ……