单选：锅炉给水经（）提高水温后，进入汽包。A．省煤器B．空气预热器C．汽包D．过热器

John was busy, ______he could not come.A but B therefore C and D nor

—Mathematics ______ study or science of numbers.A is B are C was D were

—Listen! Who ______ in the next room? The voice is so beautiful.A sings B sang C

试比较R-选择和K-选择的主要特征。

一定地段上，多物种所组成的天然群聚即为（）。