设全集为R，集合A={x|1＜x＜5 }，B={x|3≤x≤6 }．求：（1）A∩B  （2）A∪B   （3）C_RA∩B   （4）C_R（A∩B）．

Not until he retired from teaching three years ago ________having a holiday abroad.

(2012·辽宁，32)

A．he had considered

B．had he considered

C．he considered

D．did he consider