条件一 k可以被10整除 所以 2^k的尾数有可能是4，也有可能是6，也就是余数可能为4或6 不确定，不充分
条件二 k可以被4整除 所以2^k的尾数一定是6，也就是除以10后的余数一定为6.充分 所以选B
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