## Core Concept
Bernoulli's principle is a fundamental concept in fluid dynamics that describes the relationship between the pressure and velocity of a fluid in motion. It states that as the velocity of a fluid increases, its pressure decreases, and vice versa. This principle is named after Daniel Bernoulli, who first described it in the 18th century.

## Why the Correct Answer is Right
The correct answer, **C. As the velocity of a fluid increases, its pressure decreases**, accurately reflects Bernoulli's principle. This principle is based on the conservation of energy, which states that the total energy of a fluid remains constant along a streamline. As the velocity of a fluid increases, its kinetic energy increases, which leads to a decrease in its potential energy, and consequently, a decrease in its pressure.

## Why Each Wrong Option is Incorrect
* **Option A:** "As the velocity of a fluid decreases, its pressure increases" is partially correct but does not fully capture the essence of Bernoulli's principle. It is also redundant with the correct answer.
* **Option B:** "As the velocity of a fluid increases, its pressure remains constant" is incorrect because it contradicts the fundamental concept of Bernoulli's principle, which states that an increase in velocity leads to a decrease in pressure.
* **Option D:** "As the velocity of a fluid decreases, its pressure decreases" is incorrect because it is the opposite of what Bernoulli's principle states.

## Clinical Pearl / High-Yield Fact
Bernoulli's principle has significant implications in medicine, particularly in understanding blood flow and cardiovascular diseases. For example, it helps explain why aneurysms (dilations of blood vessels) can occur and why certain heart murmurs are associated with abnormal blood flow patterns.

## Correct Answer: C. As the velocity of a fluid increases, its pressure decreases.