Description should include the following causes and effects:

If water vapor in the air condenses when the carburetor temperature is at or below freezing, ice may form on internal surfaces of the carburetor, including the throttle valve. This restricts the flow of the fuel-air mixture and reduces power.
If enough ice builds up, the engine may cease to operate.
The first indication of carburetor icing in an aircraft with a fixed-pitch propeller is a decrease in engine revolutions per minute (rpm), which may be followed by engine roughness.
In an aircraft with a constant-speed propeller, carburetor icing is usually indicated by a decrease in manifold pressure, but no reduction in rpm. The propeller pitch is automatically adjusted to compensate for loss of power. Thus, a constant rpm is maintained.

PLT189, PLT190, and PLT191

Process/Skill Questions:

What causes carburetor ice?
What is sometimes the first indication of carburetor icing in an aircraft with a fixed-pitch propeller? With a constant-speed propeller?