单项选择题

She ought to stop work; she has a headache because she ___________ too long.

A．has been reading
B．had read
C．is reading
D．read
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单项选择题
Commercial application of the research finding will be possible if salad cream can be made resistant to bacterial attack （）

A．by varying its chemical composition
B．by turning it into a solid lump
C．while keeping its structure unchanged
D．while retaining its liquid form
单项选择题
The word colonies ( Line 2, Para. 4) refers to （）

A．tiny globules
B．watery regions
C．bacteria communities
D．little compartments
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