正确答案是:How many hours has the baby been sleeping?
用现在完成进行时,表示动作一直延续到现在,所以说“到现在为止”睡了几个小时。
现在完成进行时这个时态就含有“到现在为止”的意思,所以句子末尾不用再加“for now”或者“now”。
How many hours has the child been asleep.
how many hours has this child been sleeping
How many hours is the child sleeping now?
How many hours has the child been sleeping by now?
How many hours has the baby been sleeping?