已知白磷的着火点为40℃，红磷的着火点为240℃，按如图A所示的实验装置，将少量白磷和少量红磷分别放入两支试管中，塞上带有玻璃管和气球的橡皮管，然后放入盛有90℃热水的烧杯中；另将少量白磷直接投入到烧杯的热水中，观察的实验现象如下：

①试管a中的白磷燃烧，产生大量的白烟；②试管b中的红磷不燃烧；③烧杯热水中的白磷不燃烧。

如图所示，为了验证“阿基米德原理”，某同学做了如下实验：

    Tim Becker and his neighbors are doing something to make their neighborhood a safe area.

    When Tim Becker gets in his car to go shopping, he doesn't1 drive to a store and back home. He always looks 2up and down the streets of his neighborhood. He looks for anything 3 such as strange cars, loud noises, 4 windows, or people gathering on street corners.

    Tim 5 to a neighborhood watch group（巡逻小组）in Strongsville, Indiana, USA. The neighborhood watch group6 on his third Wednesday of every month. That's 7 Tim gets together with about ten of his neighbors to discuss community 8 Members of the neighborhood watch group want to help the police 9 their homes, streets, and families.

    Tina Stedman,10president of neighborhood watch group, agrees with Tim. “People seem to think that crime happens to other people but not 11them. Actually, it really happens,” she said, “and I think nobody has the12 to steal from other people or to make them feel 13 at their own homes.”

    Alex, a member of the group, said that all the neighbors were14to help one another, “We15 each other's homes. We keep watch on the neighborhood at night and on weekends. Usually a16 of four or five of us goes out together. If something doesn't look right, we will call the 17 For example, if we notice a group of teenagers who seem to be looking for 18 or someone destroying property（财产）, we report to the police.”

    Alex feels the neighborhood watch group19a lot in reducing crimes. Her husband Jim agrees, “Police are good people, but they can't do20.