21世纪人类的生存环境面临的八大挑战之一是（）。
A．传染病威胁人类健康
B．土地严重沙化
C．世界经济逐渐衰退
D．海平面水位下降
参考答案：B
He left his homeland, _________ never _________ back again.
A．determined; to come
B．being determined; to come
C．determined; coming
D．determining; coming
经济理论
