Question

They _________ in Guangzhou since 2000. A. lived B. had lived C. have lived D. were living

Answer 1

答案选c是对的，你的理解也很对。
since可以在过去完成时中出现，但需有表明是用过去完成时的条件。如：
they
said
that
she
had
lived
in
guangzhou
since
2000.在此从句中用过去完成是因为与they
said
相比，lived在said
前。
by
the
year
+过去年份、
before+过去年份（应出现在宾语从句中）与since....都是表示时间状语的方法，一般不会同时出现在一个句子中。