___ mothers leave baby pandas alone ,they will often be taken away.

C.When

Since, 表示原因。

A
在熊猫母亲让宝宝单独留下之后,宝宝们经常会被带走.

C,因为Since表示“一旦”，After表示“在```之后”。

选d
since表示原因的

since
conj.
1. (常用于现在完成时或过去完成时)自...以来,从...至今
It's been ten years since they married.
他们结婚到现在已经十年了。
She has been living a hard life since her husband died.
她自从丈夫死后,一直过着艰苦的生活。

2. 既然;因为,由于
He must have taken the book since it isn't here.
他一定是把书拿走了,因为书已不在这里了。
Since it is so hot, let's go swimming.
既然天气这么热,我们去游泳吧。