单项选择题

The brain works much more efficiently when （）.

A．both sides of the brain are used equally
B．the left side is fully developed
C．scientists work on it
D．the fight side is fully developed

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单项选择题Right now the California Air Resources Board requires that a percentage of new cars sold should be free of emissions. What is that percentage（）

A．1%.
B．5%.
C．10%.
D．100%.

单项选择题What message can we get from the last paragraph（）

A．American football might develop in different places.
B．American football is supposed to be changeable.
C．American football will have its own rules.
D．American football might see changes in the years to com