导致肺性脑病的因素有（）。

A.CO₂潴留

B.缺氧

C.酸碱平衡失调

D.脑组织pH下降

E.渗透压改变

－Hello, Mary! This is John. You must know why I _________ you.

－Oh, hello, John! I've waited for this call since last month.

A．called

B．am calling

C．was calling

D．have called