C.
对次数提问,有两种,
1.how often
2.how many times
how long有以下两个主要意思：
1.表示多长时间,主要用来对一段时间(如three days,four weeks 等)提问.如：
A：How long did he stay here?他在这儿呆了多久?
B：About two weeks.大约两个星期.
A：How long does it take to get to London from here?从这里到伦敦要多长时间?
B：At least ten hours.至少要10个小时.
2.表示某东西有多长.如：
A：How long is the river?这条河有多长?
B：About 500 km.大约500千米.
二、how often的用法
how often 指每隔多久,主要用来对频度副词或状语(如：once a week,three times a month 等)提问.如：
A：How often does he come here?他(每隔)多久来一次?
B：Once a month.每月一次.
A：How often do you visit your mother?你多长时间看你妈妈一次?
B：Once a week.一周一次
how long 是对时间多久的提问.