下列有关命题的说法正确的是 (   )

A：命题“若，则”的否命题为：“若，则”．

B：“”是 “”的必要不充分条件．

C：命题“

    Every August a fantastic sight can be seen in the skies above Ontario and Montreal﹣millions of amazing Monarch butterflies （ 帝 王 蝶） beginning their 4，000﹣kilometer journey from Canada to Central Mexico．In fact，the journey，which takes about 3months，is longer than the average （ 平 均 的 ） life of the butterfly．Finally many of the butterflies reaching their final place are the great grandchildren or even the great great grandchildren of the butterflies who began the journey in Mexico the year before．