A:He does his homework and listens to music.
B:He listens to music and cleans his room.
C:He does his homework and takes a walk.
某市社会调查队对城区内一个社区居民的家庭经济状况进行调查。调查的结果是,,该社区共有500户,高收入、中等收入和低收入家庭分别有125户、280户和95户。已知该市有100万户家庭下列表述正确的是 ( )
关于艾弗里“肺炎双球菌转化实验”和“噬菌体侵染细菌实验”的叙述中,正确的是( )
城市里有“超声波洁牙”美容店,超声波之所以能洁牙,是因为( )