平均动脉压应在多少孕周测定()。
A.孕13~20周
B.孕20~24周
C.孕14~24周
D.孕14~20周
E.孕18~24周
参考答案:C
某产品是由4个单元组成的串联系统,每个单元寿命均服从指数分布,故障率分别为λA=0.003/h,λB=0.001/h,λC=0.004/h,λD=0.002/h,则:
系统的平均故障间隔时间是( )。
A.1000h
B.100h
C.10h
D.500h
28. They ____ friends since they met in Shanghai ten years ago.
A.have become
B.have turned
C.have been
D.have grown