7．_____ of my parents is poor-educated,they both left school when they were 13.

C
all是指三个或三个以上,而both是指两个；
both of两者都
Both of them are students.他们俩都是学生.
all of三个或在三个以上都
All of us are going to see the game.我们都要去看比赛.
either of (两者其一)
neither of(两者都不)
这两个后＋单数
both of (两者都),后＋复数
all of (三者或三者以上都是)
这道题因为 is poor-educated,如果Is改成are的话肯定选A,但是是Is,只能选C了.