Professor Smith, together with his assistants, ________on the project day and night to meet the deadline. [      ]

A. work  

B. working

C. is working  

D. are working

对生产工艺允许被调参数在一个较宽的范围内波动的系统调节应选择（）。

A、比例调节规律

B、比例微分调节规律

C、比例积分调节

D、位式调节规律