1公顷=____平方米

C

    Jackson was driving back home along a road one evening. He just lost his job. It was winter and it was very cold, just like his feelings.

    An old lady with her car was standing on one side of the road. The lady looked very worried. She needed help. No one stopped to help her. Jackson stopped in front of her and got out of his car. Even when he put a smile on his face, the old lady still looked worried. Was he going to hurt her? He didn't look safe, and he looked poor and hungry.

    Jackson knew how she felt. He said." I'm here to help you, madam. Why don't you wait in my car? By the way, my name is Jackson."

    The lady thought for a while and then went into his car. There was something wrong with her car. Jackson went under her car to repair it. After a long time, he finished it. He got dirty and his hands hurt. Then the lady told him that her name was Annie. She asked Jackson how much she should pay for his help.

    Jackson never thought about getting money. This was not a job to him. This was helping someone in need. He told her that if she really wanted to pay him back, the next time she saw someone who needed help, she should give that person help.

已知矩形ABCD的长AB=4，宽AD=3，将其沿对角线BD折起，得到四面体A﹣BCD，如图所示，给出下列结论：

①四面体A﹣BCD体积的最大值为 $\frac{72}{5}$ ；

②四面体A﹣BCD外接球的表面积恒为定值；

③若E、F分别为棱AC、BD的中点，则恒有EF⊥AC且EF⊥BD；

④当二面角A﹣BD﹣C为直二面角时，直线AB、CD所成角的余弦值为 $\frac{16}{25}$ ；

⑤当二面角A﹣BD﹣C的大小为60°时，棱AC的长为 $\frac{14}{5}$ ．

其中正确的结论有____（请写出所有正确结论的序号）．

2012年2月，国务院发布空气质量新标准，下列项目没有列入空气质量监测的是（   ）

在我国的青藏高原，大部分地区水的沸点仅为84～87℃，这说明水的沸点随高度的增加而 ____（填“升高”、“降低”或“不变”）．在冬季，向保温杯中倒入半杯刚烧开的水，拧上杯盖，过一会儿，杯盖很难拧开，这主要是 ____作用的结果