已知M,N为集合I的非空真子集，且M，N不相等，若$N\cap C_{I}M=\varnothing$则$M\cup N=$（    ）

Pedro: Hi, Lily. When do you start class in the morning?

Lily: Usually at 8:00.

Pedro: It's early. We start class at 9:00 in Spain.

Lily: When do you usually eat dinner, Pedro?

Pedro: At 9:30 or 10:00 p.m.

Lily: Wow! That's too late. I always go to bed at 9:30. What do you do on the weekend?

Pedro: I often play sports with my friends. Sometimes I go shopping with my mum.

Lily: Sounds interesting. I usually clean my room. Sometimes I cook for my family.

黄土高原严重的生态环境问题是（    ）