在$△ABC$中，“$\cos A+\sin A=\cos B+\sin B$”是“$\angle C=90°$”的（    ）

    On a hot summer day in America, a little boy decided to go for a swim in the old swimming hole behind his house. He dived into the cool water, Not 1that as he swam towards the middle of the lake, a crocodile (鳄鱼) was swimming toward him.

    His mother2was looking out of the window and saw the two as they got closer and closer. In great3she ran toward the water,4 to her son as loudly as she could. Hearing her voice, the little boy became 5 and made a U-turn to swim to his mother.6 it was too late. Just as he reached her, the 7reached him. From the shore, the mother 8 her little boy by the arms just as the crocodile snatched his legs. That began an incredible tug-of-war (拔河) between the 9The crocodile was much 10 than the mother, but the mother wouldn't11A farmer 12to drive by, heard their screams, raced from his truck, took aim and13 the crocodile.

    14 after weeks and weeks in the hospital, the little boy survived. His 15were extremely scarred (留下疤痕) by the attack of the animal. And, on his arms, were deep scratches (抓痕) where his mother's fingernails dug into his flesh in her effort to hang on to the son she loved.

    The newspaper reporter, who 16the boy, asked if he would show him his scars. The boy lifted his legs. And then, with obvious 17 he said to the reporter, “But look at my arms. I have great scars on my arms, too. I have them 18my Mom wouldn't let go.”

    Never judge another person's19, because you don't know 20 they were made.

生物发光的奥秘

    说到生物世界里的发光现象，人们首先会想到萤火虫，但是除了这种昆虫外，还有许多生物也能发光。人们发现，不同的生物会发出不同颜色的光来。所有的植物在阳光照射后都会发出一种很暗淡的红光，微生物一般都会发生淡淡的蓝光或者绿光，某些昆虫会发出黄光。仔细地划分一下，生物发光可分两类：一类是被动发光，如植物，那些微弱的红光不过是没能参与光合作用的多余的光，这种光对植物是否有着生物学上的意义目前还是个谜，但一般的看法是这种光无意义，就像涂有荧光物质的材料经强光照射后再置于黑暗中发光那样；另一类是主动发光，尽管有一些主动发光的意义目前还未全部认识清楚，但有一点是可以肯定的，绝大多数生物的主动发光是有用途的。光是一种能量，主动发光是对能量的一种消耗。生物的生存策略有一个最基本的共同点，那就是在维持生命的正常活动中最大限度地去节省能量，因此主动发光必定是主动发光生物生存的一个重要手段。

    1885年，社堡伊斯在实验室里提取出萤火虫的荧光素和荧光素酶，指出萤火虫的发光是一种化学反应。后来，科学家们又得到了荧光素酶的基因。经过科学家们的研究，萤火虫的发光原理被完全弄清楚了。我们知道，化学发光的物质有两种能态，即基态和激发态，前者能级低而后者能级很高。一般地说，在激发态时分子有很高并且不稳定的能量，它们很容易释放能量重新回到基态，当能量以光子形式释放时，我们就看到了生物发光。如果我们企图使一个物体发光，我们只需要给它足够的能量使它从基态变成激发态就行了。但生物要发光则需要体内的酶来参与，酶是一种催化剂，并且是高效率的催化剂。它可以促使化学反应的发生，给发光物质提供能量，且能保证消耗的能量尽量少而发光强度尽可能高。在萤火虫体内，ATP（三清腺酸苷）水解产生能量提供给荧光素而发生化学反应，每分解一个ATP氧化一个荧光素就会有一个光子产生，从而发出光来。目前已知，绝大多数的生物发光机制是这种模式。不过在发光的腔肠动物那里，荧光素则换成了光蛋白，如常见发光水母的绿荧光蛋白，这些绿荧光蛋白与钙或铁离子结合发生反应从而发出光来。