扩大核心产品份额之争为企业间核心能力竞争的（）。
A、第一层次
B、第二层次
C、第三层次
D、第四层次
参考答案：C
Cache一致性协议
The rated operating time of a self-contained breathing device may be reduced in actual use because of（）.
A.pressure differences in the atmosphere
B.the length of the hose attached to the facepiece
C.the physical exertion of the person wearing the device
D.spaces containing poisonous vapors
