形成荒漠化的人为原因有( )
①过度樵采 ②过度放牧 ③过度农垦 ④水资源利用不当
A.①②
B.③④
C.①②③
D.①②③④
答案:D
这四项都会导致荒漠化,选D。
William found it increasingly difficult to read, for his eyesight was beginning to ________.[ ]
A. disappear
B. fall
C. fail
D. damage
为什么一般氟利昂制冷系统中要装设干燥过滤器?