牛的骨质软化症通常是由于缺（）引起的。
A.钙
B.磷
C.镁
参考答案：B
(2013·高考湖南卷)—I don’t understand why you didn’t go to the lecture yesterday afternoon.
—I’m so sorry.But I________my homework.
A．had done
B．was doing
C．would do
D．am doing
一般累犯
