构成细胞生命大厦基本骨架的化合物是[ ]
A．无机化合物
B．有机化合物
C．化学元素
D．蛋白质
答案：B
解析下面的句子
17. Trains are fast and convenient , but rush hours can be terrible.
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18. Tricycles are worth using if you want to explore the narrow alleys of old Beijing .
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《凤凰展翅》是唢呐演奏的.( )
