单选题We can never forget 

问题补充：

单选题Wecanneverforgetthedays_______thetwoastronautsinspaceshipworkedtogetherandthedays________wespenttogether.A.when; whichB.which; whenC.what; thatD.on which; when

答案：

A解析选择定语从句的关系词关键在于区别关系词在从句中作什么成分。第一个先行词the days在从句中作状语，故用when或on which；而第二个the days在从句中作spent的宾语必须用which或that。综合两个空，只能选A。