用at the same time表示转折时,前面必须加but吗?however用在句中是前后都要用逗号隔开吗?

问题描述:

用at the same time表示转折时,前面必须加but吗?however用在句中是前后都要用逗号隔开吗?
例题:The work is really difficult,____it's very interesting.
A.however B.therefore C.still D.at the same time
请说明原因,

选D.联系前后文,只有D才对.这项工作真的很难,但同时也很有趣.
at the same time不用一定加BUT的.
我记得however放在句中时后面要用“,“隔开的,放在结尾处时,前面不用”,“