寄宿制学校必须设立卫生室，非寄宿制学校可视学校规模设立卫生室或保健室。（）
参考答案：对
Though I have often heard this song ______, I have never heard you ______ it.
A．being sung; sang
B．sang; singing
C．sung; sing
D．to be sung; to sing
