Tehy eat beef (once of twice) a week.(对括号内单词提问)

How often do they eat beef a week?

how often do they eat beef a week
once or twice

这句话是不是有些拼写错误?They(不是tehy) eat beef once or(不是of) twice a week.中文意思：他们每周吃一到两次牛肉.
once or twice是频度副词做状语,对该状语提问,用How often,意为：多久一次.
所以特殊疑问句如下：
How often do they eat beef?
如果硬要对括号内词提问,则：
How often do they eat beef in a week?
这里的介词in是必须的,为避免引起歧义.