Firms that use computers have found that the number of staff ______ is needed for quality control ca

题目

Firms that use computers have found that the number of staff ______ is needed for quality control can be substantially reduced.
[A] whose [B] as [C] what [D] that
选哪个?为什么?请翻译整句

答案

选d,that引导定语从句修饰staff
大概是“使用电脑的公司已经发现高位控制的配备职员数量实质上可以减少”