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λ_A=0.003/h，λ_B=0.001/h，λ_C=0.004/h，λ_D=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