8.Studies of the human “sleep-wake cycle” have practical relevance for matters ranging from duty assignments in nuclear submarines and air-traffic control towers to the staff of shifts in 24-hour factories. (A) to the staff of (B) to those who staff (C) to the staffing of (D) and staffing (E) and the staff of the key:c A和c有什么区别，谢谢。
here is the difference: staff refer to people, say workers, clerks, ect. staffing is gerund, meaning the arrangement of the staff, eg, who comes at 6am, who comes at 9am, etc. A is wrong
