Most students today are aware of the enviromental challenges facing our planet.这里为什么用facing
问题描述：
Most students today are aware of the enviromental challenges facing our planet.这里为什么用facing
答
是动名词，facing our planet是修饰challenges的。
答
facing our planet是动名词形式做后置定语,修饰enviromental challenges.整句话可以当一个省略句,即：Most students today are aware of the enviromental challenges which face our planet.和原句意思一样.