霍乱的致病因素是（）。
A.大肠
B.小肠
C.内毒素
D.外毒素
E.环境
参考答案：D
低压隔爆开关进线接线嘴紧固程度螺旋线咀用（）为合格，压叠式线嘴用（）为合格。
He was late for the meeting because the bus he took _____ halfway. [ ]
A. broke in
B. broke down
C. broke out
D. broke up
