未经中间试验就转化为生产的技术绝不能冒险采用，这体现了建设方案技术比选的（）原则。

A.先进性

B.实用性

C.可靠性

D.安全性

----How many times have you been to China?

--- In fact, it is the third time that I __________ here.

A．was

B．am

C．had been

D．have been