It's about three years since they_here.是填came还是have been?

问题描述:

It's about three years since they_here.是填came还是have been?

选camesince的四种用法 1) since +过去一个时间点(如具体的年、月、日期、钟点、1980,last month,half past six).例如:I have been here since 1989.1989起,我一直在这儿.2) since +一段时间+ ago.例如:I have...