Most voluntary organizations have only a small full-time ___, and depend very much on volunteers and part-timers.(答案為A,)

A team   B  number    C staff     D crowd

 解釋里說: staff是集體名詞,前面不加"定冠詞" (解釋里不知道是不是少了"不"一字)。可是有人說看過跟前面帶a  的staff短語, 所以在這里想請教一下該題的解釋以及staff的用法。