土壤的自净作用有哪几种?
参考答案:物理自净作用、物理化学自净作用、化学自净作用、生物自净作用.
Professor Smith, along with his assistants, _____ on the project day and night to meet the deadline.
A.work
B.working
C.is working
D.are working
下列哪些是发动机的动力性指标:()
A、升功率
B、有效转矩
C、有效功率
D、发动机转速
E、平均有效压力