目前，我国证券交易所的证券回购券种主要是（　　）。

A．国债

B．股票

C．可转换债券

D．基金证券

A crew member reports that the high-pressure alarm light of a low-pressure CO₂ fixed fire extinguishing system is illuminated. The most probable cause of this condition would be that（）.

A.an air leak has developed in the tank

B.the tank cooling system has malfunctioned

C.the pilot cylinder discharge valve is leaking

D.an excessive amount of insulation has been installed on the tank and piping