He made another wonderful discovery, ________ of great importance to science．

which I think is

which I think it is

which I think it

I think which is

Happy birthday, Alice! So you have ________ twenty-one already!

become

turned

grown

passed

The accident is reported to have occurred ________ the first Sunday in February．

at

on

in

to

Mr．Smith used to smoke ________ but he has given it up．

seriously

heavily

badly

hardly

When we plan our vacation, mother often offers ________ suggestions．

careful

practical

effective

acceptable

They spent as many hours as they could ________ their lessons．

to study

study

studying

studies

It is generally considered that terrorism is ________ danger to ________ society．

a; the

/；the

/;/

a;/

Professor Smith had not given ________ talk on Shakespeare for a long time, so he had to brush up on some of ________ plays．

/;/

a; the

the;/

the; the

While he was investigating ways to improve the telescope, Newton made ________ discover which completely changed ________ man’s understanding of color．

a;/

a; the

/; the

the; a

For a long time they walked without saying ________ word．Jim was the first to break silence．

the; a

a;

the

a;/

the;/