1.D.on earth
2.B.say
3.A.native
4 B.subjects
5.C.why
6.C.x
7.B.for
8.A.when
9.B.everything
10.C.they go to school
11.D.more
12.D.at（不确定 我觉得应该是归根结底这个意思,不过at all很少用于陈述句）
13.B.how to learn
14.C.whenever
15.B.himself
16.C.other
17.B.either
18.B.do
19.D.but
20.B.to learn
不保证全对