A model or an experiment in a science class can help us see things which would       remain hidden．

 Process has been very smooth. ______, we are not confident that it will be completed on time.

—Are you going to Dalian by air?—No, the tickets are ____expensive for me. 

The poor boy couldn't work the problem out. He wasn't ____ clever.

A person, when he or she is young, can’t read __________many books.