I didn't know you were in trouble at that time,________I would have given you a hand.[ ]

A．whereas  

B．however  

C．otherwise  

D．therefore

氧瓶使用后，瓶内应保留1kg/cm²的剩余压力，目的在于（）。

A.表明瓶未做过其他用途

B.外界杂质不易进入瓶内

C.再充气时，瓶无需清洗

D.保护减压器不易损坏

E.备取样验证气体性质