单项选择题

A．They have already existed.
B．They will come into being in the future.
C．They are only people’s unrealistic dreams.
D．They may be put into commercial use very soon.
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单项选择题
A．The 2:00 train will arrive earlier.B．The 2:30 train has a dining car.C．The woman prefers to take the 2:30 train.D．They are going to have some fast food on the train.
A．The 2:00 train will arrive earlier.
B．The 2:30 train has a dining car.
C．The woman prefers to take the 2:30 train.
D．They are going to have some fast food on the train.
单项选择题
A．It keeps blood vessels from being blocked.B．It speeds up their recovery after surgery.C．It increases the blood flow to the heart.D．It adjusts their blood pressure.
A．It keeps blood vessels from being blocked.
B．It speeds up their recovery after surgery.
C．It increases the blood flow to the heart.
D．It adjusts their blood pressure.
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