互补功率放大电路是一种无输入、输出变压器的推挽功率放大电路，互补对称推挽功率放大电路中采用（）和（）三极管各一只。
参考答案：PNP型 NPN型
目前认为基因表达调控的主要环节是（）
A.基因活化
B.转录起始
C.翻译起始
D.转录后加工
E.翻译后加工
Most covered lifeboats will right themselves after capsizing if the（）.
A.passengers are strapped to their seats
B.fuel tanks are not less than half full
C.sea anchor is deployed to windward
D.lower ballast tanks are filled with water
