we usually work-_____-- eight hours on weekdays

we usually work-_____-- eight hours on weekdays
we usually work-_____-- eight hours on weekdays

we usually work-_____-- eight hours on weekdays
填for,因为for+一段时间,而eight hours就是表示一段时间,如果是一个时间点那么就用at,比如eight o'clock .
表示时间前的只能用这两个词语,而用的时候,要根据时间点和一段时间来确定.