单项选择题

Many scientists have been probing psychological problems．（）

A．solving
B．exploring
C．settling
D．handling

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单项选择题
A great deal has been done to remedy the situation．（）

A．maintain
B．improve
C．assess
D．protect
单项选择题
John is collaborating with Mary in writing an article．（）

A．cooperating
B．competing
C．combining
D．arguing
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