What's wrong _____________ your child? [ ]
A. with
B. in
C. at
D. about
答案：A
若xn-3•xn+3=x10，则n=______．
汽车零件与构件产生断裂往往由疲劳引起，疲劳破坏占断裂事故的（）
A、50%-60%
B、60%-70%
C、70%-80%
D、80%-90%
