A:设法提高牛、羊的环境容纳量
B:保持草原生态系统的稳定
C:调整能量流动方向,使能量流向对人类最有益部分
D:实现对能量的分层次多级利用,使废物资源化
①同学之情 ②朋友之情
③师生之情 ④亲子之情
—Would you please help me _______, Wang Fei? This question is too hard for me.
—OK. Let me have a try.