Although the causes of cancer ________, we do not yet have any practical way to prevent it．

are being uncovered

have been uncovering

are uncovering

have uncovered

It ________ long before Chinese ________ on the moon．

will not be; will land

is; will land

will not be; lands

is; lands

In the last few years, a few more theme parks ________ in our cities．

have opened

had been opened

have been opened

were opened

Those books which ________ often ________ well．

are well received; are sold

are well received; sell

well receive; sell

were received; sell

All the preparations for the task ________, and we are ready to start．

completed

to be completed

are to complete

have been completed

In the spring sun, local residents ________ Chengdu Botanical Garden where the Spring-Greeting Flower Show is on．

are to attract to

are to be attracted to

are attracted to

are attracting to

With the development of science, more new technology ________ to the fields of IT．

has introduced

is being introduced

is introduced

was introduced

More patients ________ in hospital this year than last year．

treated

have treated

had been treated

have been treated

All the employees except the manager ________ to work online at home．

encourages

encourage

is encouraged

are encouraged

The first use of atomic weapons was in 1945, and their power ________ increased enormously ever since．

is

was

has been

had been