斜导柱侧抽芯注射模中楔紧块的作用是（）。

A、承受侧向推力

B、模具闭合后锁住滑块

C、定位作用

D、A和B正确

— You look unhappy, my son?

— I'm wondering how long it'll be          we come to China again. [ ]

A. since

B. when

C. before

D. after