许多自然灾害,特别是等级高、强度大的自然灾害发生以后,常常诱发出一连串的其他灾害接连发生,这种现象是()
A.原生灾害
B.次生灾害
C.衍生灾害
D.灾害链
参考答案:D
By the time of 1969, there were many space explorations.
A.have been
B.must be
C.had been
D.must have been
Kate could hum songs and difficult _______ . [ ]
A. piece of musics
B. pieces of musics
C. piece of music
D. pieces of music