It's a long time_____we met lastA.so B.after C.since D.before请告诉我答案,顺便说明原因,谢谢

It's a long time_____we met last
A.so B.after C.since D.before请告诉我答案,顺便说明原因,谢谢

c,'so' cant leat a subordinate clause,"after" is as same as previous one."before" is obviously illogical.
just for your reference

答案是C
习惯用法。