生物群落演替
参考答案：生物群落演替：生物群落随时间推移，一些物种消失，另一些物种侵入，出现了生物群落及其环境向着一定方向有顺序的发展变化过程。
If the United States had built more homes for poor people in 1955, the housing problems now in some parts of the country () so serious.
A. wouldn’t be
B. wouldn’t have been
C. will not be
D. would have not been
集体土地未经依法征收( )。
A．不得将集体土地用于商业性房地产开发
B．集体土地可以用于商业性房地产开发
C．可以转让
D．可以挪作他用
