厌氧培养时，厌氧环境的微生物质量控制需要下列哪类细菌（）
A.专性厌氧菌
B.专性需氧菌
C.微需氧菌
D.兼性厌氧菌
E.嗜二氧化碳菌
参考答案：B
The number of college graduates ________ surprisingly large and a large number of them ________
unable to find a proper job. [ ]
A. is; is
B. are; are
C. are; is
D. is; are
潜水艇潜入水下航行时，所受浮力______（选填“增大”、“减小”或“不变”），它是通过改变自身的______来实现上浮和下潜．
