慢性肺源性心脏病发生的关键环节是（）.

A.肺间质纤维化

B.肺气肿

C.肺动脉高压

D.肺动脉分支血栓栓塞

E.肺阻塞性通气障碍

In handing break bulk hazardous materials , it is forbidden to use（）.

A.cargo nets

B.metal bale hooks

C.pallets

D.slings