答案选C
every+ 基数词+ 复数名词；
every+ 序数词+ 单数名词
但是each是没有这种用法的.所以A和B都错.
其实every two weeks ,every second week 都做“每隔一周”讲.
但在实际应用中人们都用every other week来表示这一意思.
every two weeks也有人讲.而every second day 则少用.
没有every another week 的说法.