A. can not be ignited by a spark unless the vapor is in a confined space B. are nontoxic in all concentrations C. can not be ignited by a spark unless they are at their auto-ignition temperature D. are heavier than air
A. 3天内 B. 3周以上 C. 3天至3周 D. 4周以上
A. 功率管 B. 三极管 C. 晶闸管 D. 复合管
A. 成本 B. 时间 C. 费用 D. 资源