The profit-sharing plan is designed to_______the staff to work hard.

A.   calculate

B.   demonstrate

C.   speculate

D.   motivate

So far scientists have only acquired_________understanding of the physical processes that cause earthquakes.

A.   a partial

B.   a changeable

C.   an original

D.   an individual

The room was quiet，_________the occasional coughing from my throat.

A.   in case

B.   except for

C.   by far

D.   instead of

You’d better leave things________if you don’t know how to deal with them.

A.   lonely

B.   only

C.   lone

D.   alone

Scientists doubt whether it’s_________to regulate one’s biological clock by drinking.

A.   feasible

B.   incredible

C.   fashionable

D.   valuable