单项选择题

______that the earth was flat

A．Used it to be thought
B．Did it used to be thought
C．Was it used to being thought
D．Does it used to be thought
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单项选择题
The ozone layer plays as great a role in the stability of spaceship Earth as______ the waters of its lakes, ponds, oceans, rivers, and streams.
A．do
B．does
C．play
D．are
单项选择题
The major reason why Americans enjoy an abundant food supply is that the arable land at their disposal for food production is______.
A．three times more the world average
B．three times as much the world average
C．three times the world average
D．the world average is three times
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