I did not answer the door even though I knew it was my friend.

A. would be 

B. was  

C. had been  

D. were

A. degree

B. certificate

C. diploma

D. qualification

A. enthusiasm

B. experience

C. certificate

D. concentrate

A. they keep the users alert all the time.

B. they are indispensable in everyday communications.

C. they make them look more stylish.

D. they cannot be replaced by regular phones.

A. Laser is a device which concentrates light waves into an intense light beam.

B. The laser is a device which concentrates light waves and an intense light beam.

C. Laser is a device concentrates light waves into an intense light beam.

D. The laser is a device to concentrate light waves to an intense light beam.

A. which

B. what

C. when 

D. where