照样子，把句子写具体。

例：小溪流向远方。  清澈的小溪缓缓地流向远方。  

1．同学们听老师讲课。                                                                                                             

2．小鸟站在大树上唱着歌。                                                                                                      

3．天上飘着白云。                                                                                                                     

现有A、B、C、D四个互斥方案，寿命相等，有关数据如下表所示，若基准收益率i_c=15%，试问哪一个方案最好( )。

A．A
B．B
C．C
D．D