The correct answer is: D. can be used to measure gas or liquid flow rates.
A venturi tube is a device used to measure the flow rate of a fluid. It consists of a constricted section, called the throat, between two wider sections, called the inlet and outlet. The fluid flowing through the venturi tube accelerates as it passes through the throat, causing a drop in pressure. This pressure drop can be measured to determine the flow rate of the fluid.
Venturi tubes can be used to measure the flow rate of both gases and liquids. They are typically made of metal, but can also be made of plastic or glass.
Option A is incorrect because venturi tubes are not extremely difficult to use. They are relatively simple devices that can be easily installed and operated.
Option B is incorrect because venturi tubes can be made of a variety of materials, including metal, plastic, and glass.
Option C is incorrect because venturi tubes are not used to measure the presence of a liquid. They are used to measure the flow rate of a liquid.
Option E is incorrect because venturi tubes can be used to measure the flow rate of both gases and liquids.