有一读者问：有这样一道题：
This is the third week_________the dustmen haven't come for the rubbish.
A. that B. which C. when D. on which
应如何选择？
应选 that, 这是最自然的用法。 that 在此是关系副词，在从句中作状语用；和许多可省去 that 的情况一样，这里的 that 也可以省去。 when 有点别扭。 on which 不可用，但可用 during which，不过比较正式。
