在各种支架支护的巷道中，每个检查点应选（）个测点。
A.3
B.5
C.6
D.10
参考答案：C
We didn’t know you were in trouble at that time, _______ we would have given you a hand.
A．therefore
B．unless
C．otherwise
D．so
引起呼吸系统疾病最常见的病因是
A．吸烟B．肿瘤C．感染D．变态反应E．理化因素
