问题
选择题
--What are you doing now?
-- I’m talking to Mary online. She ___ from Canada a few days ago.
A．is coming back
B．come back
C．came back
D．went back
答案
答案：C
考查固定短语和时态。句子中有a few days ago，故用一般过去时，而come back固定短语返回。故选C。
D．此过程中系统的电势能减少