Tom had to buy ---these CDs because he didn't know which one

选A.
因为不知选哪个,只好全买下来.从意思上是说得通的.
无法确定一共有几张CD,因此排除D
C项each表示每一个,不妥,应该说所有的,排除.
B项从意思上确实讲得通,但用none时,它所修饰的人或物必须要有一个范围,并且要有介词of,如:
None of my friends come to see me.
如果要表达他没买,必须在后面加of.
相信我,这题我做过好几次.