representatives discussed some of the most problems facing o

epresentatives discussed some of the most (serious) problems facing our planet.
=representatives discussed some of the most (serious) problems that our planet were facing.
注：
1.face可以作“面临”,也可以作“使面临”讲,所以改过来之后不需要变成被动语态(改了反而不对).
2.前面有最高级,定语从句的引导词不能用which,要用that.
3.problems前肯定掉了一个形容词,我加了个serious