多少孕周前不能做出前置胎盘的诊断（）
A.10周
B.18周
C.28周
D.30周
E.35周
参考答案：C
His grandfather sometimes _____ him on Sundays. [ ]
A. is going to see
B. is coming to see
C. comes to see
D. go to see
一般要求钻井液在钻杆内呈以下哪种状态最好。（）
A、层流
B、紊流
C、过渡型
D、半顿型
