单项选择题

闭式热水供热系统，补给水泵宜选（），可不设备用泵，正常时一台工作，事故时两台全开。 A.
一台 B.
两台 C.
三台 D.
四台

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向左、右变方向运球时，手按拍球的右、左侧上方，使球向前反弹的速度与人跑的速度一致。

答案：正确

单项选择题

The individuals of different colors and ethnic groups are distinct and recognizable but together they create a complete picture of unity that is uniquely American. So perhaps it's better to describe the United States as a _______.
A、mosaic
B、melting pot
C、salad bowl
D、iceberg