为什么保养作业时必须切断动力源？
参考答案：保养作业时必须切断动力源，防止误操作，导致人员触电。
After a _____ discussion, they came up with a new ____ to the study of physics. [ ]
A. two hours', access
B. two hours', way
C. two-hour, approach
D. two-hour , method
简述EXW条件下买方义务。
