John and I _____ friends for eight years. We first got to know each other at a Christmas party. But we ____ each other a couple of times before that.

A．had been; have seen

B．have been; have seen

C．had been; had seen

D．have been; had seen

某计算机系统的可靠性结构是如下图所示的双重串并联结构，若所构成系统的每个部件的可靠度为0.9，即R=0.9，则系统的可靠度为（）

A．0.9997

B．0.9276

C．0.9639

D．0.6561