帷幕造孔中出现的塌孔事故主要原因是（）。
A.流沙含水量大
B.泥浆指标不符合要求
C.泥浆柱太低
参考答案：B, C
专供外用，不可内服的药物是
A．土荆皮
B．炉甘石
C．升药
D．硫黄
E．砒石
–Were you at school when he came to see you?
–Yes, I ______ a math class.
A. was having B. had C. am having
