—We'll go to Qing Yuan Mountain tomorrow. Why _____ join us?

—That's a good idea. [ ]

A. not

B. don't

C. can't

D. didn't

在信息系统开发的测试阶段，必须要用户参与的是（）。

A.单元测试

B.子系统测试

C.验收测试

D.系统测试