I won’t tell the student the answer to the math problem until he ________ on it for more than an hour．

has been working

will have worked

will have been working

had worked

I was giving a talk to a large group of people, the same talk I ________ to half a dozen other groups．

was giving

am giving

had given

have given

The young girl sitting next to me on the plane was very nervous．She ________ before．

hasn’t flown

didn’t fly

hadn’t flown

wasn’t flying

Police are now searching for a woman who is reported to ________ since the fold hit the area last Friday．

have been missing

have got lost

be missing

get lost

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

are being uncovered

have been uncovered

are uncovering

have uncovered

I ________ along the street looking for a place to park when the accident ________．

went; was occurring

went; occurred

was going; occurred

was going; had occurred

Although medical science ________ control over several dangerous diseases, what worries us is that some of them are returning．

achieved

has achieved

will achieve

had achieved

Ladies and gentleman, please fasten your seat belts．The plane ________．

takes off

is taking off

has taken off

took off

The construction of the two new railway lines ________ by now．

has been completed

have been completed

has completed

have completed

My friend, who ________ on the International Olympic Committee all his life is retiring next month．

served

is serving

had served

has served