what和thatFirmsthatusecomputershavefoundthatthenumberofstaffisneededforqualitycontrolcanbesubstantiallyreduced.[A]whose[B]as[C]what[D]that为什么选that?不用what的理由是什么?

Question

what 和that
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
为什么选that?不用what的理由是什么?

Answer 1

that 引导的定语从句,is needed for quality control 是解释说明staff的.