下列设备中，哪一个不是阻火设备？（）

A.水封井

B.安全阀

C.单向阀

D.阻火器

Financial derivatives contribute to market completeness by allowing traders to do all of the following EXCEPT :()

A. increase market efficiency through the use of arbitrage. 

B. hedge positions in other assets and engage in high risk speculation. 

C. narrow the amount of trading opportunities to a more manageable range.