Many kinds of help ________ by plane or by train to the area ever since the earthquake hit it.

A．had been offered

B．have been offered

C．are offered

D．were offered

电极（焊丝）前倾时，电弧力对熔池液体金属后排作用减弱，熔池底部液体金属（）了，阻碍了电弧对熔池底部母材的加热。