the twin sisters have learned a lot ____they came to China A when B as soon as C since D after

the twin sisters have learned a lot ____they came to China A when B as soon as C since D after
the twin sisters have learned a lot ____they came to China A when B as soon as C since D after

the twin sisters have learned a lot ____they came to China A when B as soon as C since D after
C

c.since用完成时

D

选C。这算是现在完成时态的其中一种常见结构 ：现在完成时+since+过去时，表示“自从过去的某个时刻开始，就......”，如：I've worked here since two years ago.