缩聚物

下列各式计算正确的是（　　）

A．2a+3b=5ab

B．12x-20x=-8

C．5+a=5a

D．6ab-ab=5ab

On one hand, tons of milk_______; on the other hand, tens of thousands of people ______hunger.         [      ]

A. went bad; died of      

B. went badly; died of

C. went bad; died from      

D. went badly; died from

请简述说明系统从异常处理程序中返回需要执行哪些操作。

钢轨焊接焊接作业轨温应不低于（）℃，且应避免大风和雨雪等不良天气。