前置胎盘的发生与下列哪项因素关系最小()
A．多次刮宫
B．双胎妊娠
C．子宫内膜炎
D．妊高征
E．受精卵滋养层发育迟缓
参考答案：D
Tom didn't _____ his primary school classmate Mary until he listened to her self-introduction. [ ]
A. know
B. find
C. recognize
D. realize
计算（a3）2的结果是 [ ]
A．a6
B．a9
C．a5
D．a8
