Small pipes restrict water flow because of:

excessive curvature
insufficient drop
internal friction

The correct answer is internal friction.

Explanation:

The pressure drop that occurs in a small pipe is proportional to the velocity of the fluid and the length of the pipe section. While it is inversely proportional to the diameter.

This is because the friction between the fluid and the walls of the pipe is greater when the diameter is smaller, since the friction between the water and the walls of the pipe increases.

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