按照“中国民用航空总局维修差错调查表（RI）”规定，下列各项中哪一项属于诱因（）
A、安装不当
B、勤务缺陷
C、设备工具
D、修理不当
参考答案：C
已知|a+1|+（b-2）2=0，那么单项式-xa+byb-a的次数是多少？
We didn’t know you were in trouble at that time; _______ we would have given you a hand.
A．otherwise
B．therefore
C．yet
D．however
