重大危险源是指长期地或临时的生产、加工、搬运、使用或贮存危险物质，且危险物质的数量等于或超过临界量的单元
参考答案：对
大多数UPS采用两个反向并联的（）组成一个静态开关。
A.晶体管
B.二极管
C.晶闸管
D.三极管
Nortel Industries has a preferred stock outstanding that pays (fixed) annual dividends of $3.75 a share. If an investor wants to earn a rate of return of 8.5 percent, how much should he be willing to pay for a share of Nortel preferred stock（）
A. $44.12.
B. $31.88.
C. $37.50.
