The correct answer is: D. difference of pressure between two points.
A piezometer is a device used to measure the pressure of a fluid in a closed system. It is typically a small tube that is inserted into the fluid and connected to a manometer, which is a device that measures pressure. The piezometer measures the difference in pressure between the fluid and the atmosphere, and this difference is then used to calculate the pressure of the fluid.
Piezometers are often used to measure the pressure of water in pipes or channels. They can also be used to measure the pressure of other fluids, such as oil or gas. Piezometers are a valuable tool for engineers and scientists who need to measure the pressure of fluids in a variety of applications.
Option A is incorrect because a piezometer is not used to measure the pressure in pipes, channels, etc. A manometer is used to measure the pressure in pipes, channels, etc.
Option B is incorrect because a piezometer is not used to measure atmospheric pressure. A barometer is used to measure atmospheric pressure.
Option C is incorrect because a piezometer is not used to measure very low pressures. A vacuum gauge is used to measure very low pressures.