When a wave passes from one medium to another ................ of the wave varies but its ............... remains constant.

Options

A. frequency, velocity
B. velocity, frequency
C. frequency, wavelength
D. velocity, wavelength

Explanation

To solve the question "When a wave passes from one medium to another, ................ of the wave varies but its ............... remains constant," we need to analyze the behavior of waves as they transition between different media.

### Step-by-Step Solution:

1. **Understanding Wave Properties**: When a wave travels through different media, certain properties of the wave can change. The primary properties to consider are velocity, frequency, and wavelength.

2. **Wave Equation**: The relationship between these properties is given by the equation:
\[ v = f \lambda \]
where \( v \) is the wave velocity, \( f \) is the frequency, and \( \lambda \) is the wavelength.

3. **Frequency and Medium Change**: When a wave passes from one medium to another, the frequency of the wave remains constant. This is because frequency is determined by the source of the wave and does not change when the wave enters a new medium.

4. **Velocity Change**: The velocity of the wave, however, does change when it enters a different medium. This is due to the different properties of the media, such as density and elasticity, which affect how fast the wave can travel.

5. **Wavelength Adjustment**: Since the frequency remains constant and the velocity changes, the wavelength must also adjust according to the wave equation. If the velocity increases, the wavelength increases, and if the velocity decreases, the wavelength decreases.

6. **Filling in the Blanks**: Based on the analysis:
- The first blank, which refers to the property that varies, is "velocity."
- The second blank, which refers to the property that remains constant, is "frequency."

Thus, the complete statement is: "When a wave passes from one medium to another, **velocity** of the wave varies but its **frequency** remains constant."

### Final Answer:
**Velocity, Frequency**