不考虑个人的利益，才能实现人生的意义。（   ）

如果把一个奇数位的自然数各数为上的数字从最高位到个位依次排列，与从个位到最高位依次排列出的一串数字完全相同，相邻两个数位上的数字之差的绝对值相等（不等于0），且该数正中间的数字与其余数字均不同，我们把这样的自然数称为“阶梯数”，例如自然数12321，从最高位到个位依次排出的一串数字是：1，2，3，2，1，从个位到最高位依次排出的一串数字仍是：1，2，3，2，1，且|1﹣2|=|2﹣3|=|3﹣2|=|2﹣1|=1，因此12321是一个“阶梯数”，又如262，85258，…，都是“阶梯数”，若一个“阶梯数”t从左数到右，奇数位上的数字之和为M，偶数位上的数字之和为N，记P（t）=2N﹣M，Q（t）=M+N．

近年来在一些重大比赛中，个别运动员采用不正当的竞争手段服用兴奋剂。下列哪种物质最有可能是兴奋剂的成分

阅读下面短文，从短文后各题所给的A、B、C、D四个选项中，选出最佳选项。

    Mr. Black was over fifty years old. He sometimes made some mistakes in his work, and he lost his work. So he had 1 time every day to do something. Then he often met his old friends under a big tree. They had a drink there and 2 anything they liked.

    One day, some old men were sitting together again and talking about young people in this world. They all agreed that the old people were 3 than young people. Then one of the old 4 said that young men were stronger than old men.

    5of them agreed that this was true, 6 Mr. Black didn't. He said, "No, I am as strong now as when I was a young man." His friends were surprised 7 what he said.

    "Well," said Mr Black. "Near my house there is a big stone. When I was a young man, I used 8 to move it. But I couldn't because I was not 9. I am an old man now, and when I try to move it, I still 10. So I am as strong as when I was young. "

杨树的叶片较大，但上下相邻的叶片互不遮挡，每片叶都尽可能多接收到阳光的照射，这种现象可说明（   ）