Trees are useful to man in three important ways: they give him wood and other useful things, they give him cool places, and they help to stop drought and flood. Unluckily, in many parts of the world
Even before there were people, there were cases of air pollution. There were dust storms(沙尘暴). Volcanoes erupted, sending ash and poisonous gases into the atmosphere. When people appeared on t
，
，且
，则
的最小值为（ ）
，故选C.