单项选择题

This is so difficult a problem that the scientist is known to ______ on it for about ten years, but hasn't found an answer yet.

A．have worked
B．have been working
C．be working
D．work on
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单项选择题
Suppose there were a flood or an earthquake and the people could not be reached by trains or trucks. ______ packages of food could be dropped from planes, the people might starve.
A．Until
B．Before
C．Unless
D．If only
单项选择题
If ever again ______ happens an accident like this, we will have only ourselves to blame.
A．it
B．so
C．there
D．that
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