阅读理解，请选择正确的答案。 There are over three million known compounds, with no end(没有尽头) in sight as to the number that can and will be prepared(制备,准备) in the future. Each compound is unique(唯一的) and has characteristic(特有的) physical and chemical properties(性质). Let us consider in some detail two compounds—water and mercuric(汞,水银) oxide. Water is a colorless, odorless(无嗅), tasteless (无味) liquid that can be changed to a solid, ice at 0℃ and to a gas, steam(蒸汽) at 100℃. It is composed of two atoms of hydrogen(氢) and one atom of oxygen per molecule (分子), which represents(表现) 11.2 percent hydrogen and 88.8 percent oxygen by mass (质量). Water reacts chemically with sodium (钠) to produce hydrogen gas and sodium hydroxide(氢氧化物), with lime (石灰) to produce calcium (钙) hydroxide, and with sulfur(硫) trioxide(三氧化物) to produce sulfuric(硫的,硫磺的) acid(酸). No other compound has all these exact(精确的) physical and chemical properties; they are characteristic of water alone. Mercuric oxide is a dense(致密的), orange-red powder composed of a ratio of one atom of mercury to one atom of oxygen. Its composition(组成) by mass is 92.6 precent mercury and 7.4 percent oxygen. When it is heated to temperatures greater than 360℃, a colorless gas, oxygen, and a silvery(银子似的；有银色光泽的) liquid metal, mercury, are produced. Here again are specific(明确的,特定的) physical and chemical properties belonging to mercuric oxide and to no other substance. Thus, a compound may be identified(区别,识别,鉴定) and distinguished(区分) from all other compounds by its characteristic properties.This passage suggested that ( ).