------You’ve left the light on.
------Oh, so I have. _____________to turn it off.

--How did she get infected?
--She ________the virus without any protection.

– Where does your brother work, Jane?
-- In America. But he __________in China for 5 years.

Experiments of this kind ___in both the U．S and Europe well before the Second World War．

—I remember I left my laptop in the study last night. Why ________ here?
—Oh, I thought you might need it so I brought it here for you.

Japan’s economy _______ in the past twenty years. No one knows how long it ______ before it regains its increase.

When Mary came, I _____ on the piano.

He will have learned English for eight years by the time he        from the university next year．

Computers ____ in offices and homes since 1970s. 

--- Do you want a lift home?
--- It’s very kind of you , but I have to work late in the office . I over slept this morning because my alarm clock _________ .