为了庆祝“党的十九大”顺利召开，重庆市第十八中学初2019级某班开展了《红星照耀中国》读书活动。下面是某同学的读书分享发言，请你将它补充完整：
— Where's Mr. Yu, do you know?
— Well, it's hard to say. But I saw him a football game just now.
黄土高原主要的生态环境问题是（ ）
