The customers often complained ___they have bought from that shop

问题描述:

The customers often complained ___they have bought from that shop
that
which
what
where

what
先看从句thay have bought from that shop 不是一个完整的句子,所以应该用which,但是complain 少一个介词about(complain about是固定搭配)
所以要选what(what=about which)