You were working too hard. You’d better keep a ________between work and relaxation.

I like the sofa we’ve found in the shop. In my opinion, it is a perfect ________ for the furniture (家具) in our sitting room.

 Don’t try to persuade your boss；he won’t have the       of employing me—a fresh student. 

After years of research, he found the key ____ differences between species.

There is no _______in looking further into this case, I think.