问题补充：
--Where have you all day?
--- I came back at noon and I in this office since then.
A. been in; had been
B. been in; was
C. been; have been
D. been; am
答案：
A. been in; had been
B. been in; was
(答案→)C. been; have been
D. been; am
解析：句意：你整天都在哪里？我在中午回来了，自从那时后就一直呆在办公室里。句中有since句中用现在完成时，故选C
