They have hired a babysitter, _________we don't have.
A. who B. whom C. which D. that
(1)_________we don't have = _________we don't have hired
(2)_________we don't have 是非限制性定语从句。所以 D. that 是错误的。
(3)非限制性定语从句缺少宾语, who 和 whom 都可以作宾语。因为这是单项选择题,所以答案应该是 C. which (把 babysitter 看作是一种职业而不是具体的个人)。