It is generally accepted that people benefit considerably, both physically and mentally, from doing exercise; they need only to spend a certain time on regular exercise every week.
; 分号 请问怎么使用 在文中是什么意思？ 还有 a certain time 为什么有 a， 作为时间来讲time是单数，但是不可数吧？作为次数讲，time可做复数，才可以用 a time, many times 对吧？为什么这里用 a
