大功率高频电源电路的逆变部分通常采用（）

A.晶闸管逆变电路

B.晶体管振荡电路

C.电子管振荡电路

D.场效应管振荡电路

The settlement price for a futures contract is:()

A. an average of the trade prices during the ’ closing period’ 

B. the price of the last trade of a futures contract at the end of the trading day. 

C. the price at which all trades over a certain period are executed.