Mercury is often used in barometer because A. it is the best liquid B. the height of barometer will be less C. its vapour pressure is so low that it may be neglected D. both (B) and (C)

The correct answer is D.

Mercury is often used in barometers because it has a high density and a low vapor pressure. This means that it is heavy and will not evaporate easily, which makes it ideal for measuring air pressure.

The height of a barometer is determined by the difference in pressure between the air at the top of the barometer and the air at the bottom. The higher the air pressure, the shorter the barometer will be. Mercury is a dense liquid, so it will sink to a lower level in a barometer than a less dense liquid, such as water. This means that a mercury barometer will be shorter than a water barometer for the same air pressure.

The vapor pressure of a liquid is the pressure exerted by the vapor of the liquid in equilibrium with the liquid. The vapor pressure of mercury is very low, which means that very little mercury vapor is present in the air above the mercury in a barometer. This is important because the vapor of a liquid can interfere with the measurement of air pressure.

For these reasons, mercury is the ideal liquid to use in a barometer.