题目

It happened--whether by accident or ____ --that the two of them were left alone after all the others had gone. A. plan        B. design         C. purpose           D. control  答案：B下面的实例中，能构成群落的是(　　) A．亚马孙河谷的热带雨林 B．大兴安岭的红松林 C．无菌培养基污染后长出的共生菌落 D．无菌培养基上接种后长出的大肠杆菌菌落