Every possible means ____ to save the wounded soldier up to now.

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A. was tried

B. were tried

C. have been tried

D. has been tried

在风险一收益二维平面上，不考虑做空机制，由风险证券A和无风险证券B建立的证券组合一定位于( )。

A．连接A和B的直线上
B．连接A和B的直线的延长线上
C．连接A和B的曲线上，且该曲线凹向原点
D．连接A和B的曲线上，且该曲线凸向原点