40. In nature, among metals, the one that is liquid at room temperature is $\qquad$
A. Cesium
B. Mercury
C. Gallium
D. Rubidium
【Answer】B
【Analysis】Mercury, a chemical element, is located at position 80 in the periodic table. Commonly known as quicksilver, its elemental symbol is $\mathrm{Hg}$, and it is the only metal that exists as a liquid at room temperature and pressure in nature. Mercury is a silver-white, shiny, heavy liquid, with stable chemical properties, insoluble in acid and alkali. Mercury can evaporate at room temperature, and mercury vapor and mercury compounds are highly toxic, often causing chronic poisoning. Mercury has a long history of use and a wide range of applications. Mercury is widely present in nature, and trace amounts of mercury are generally found in most living organisms, so there are trace amounts of mercury in our food, which can be metabolized through excretion, hair, etc. Cesium, gallium, and rubidium are low-melting-point metals that can also be liquid at room temperature, but in nature, they do not exist as metallic elements but are all present as compounds. Therefore, the correct choice is B.