The doppler effect is a change in the frequency of sound caused by the motion of either their listener or the sound source.

The correct answer is an apparent.

Explanation:

Doppler effect is a phenomenon in which there is a increase or decrease in the frequency when there is relative motion between an observer and the source.

When the observer and the listener are moving towards each other then there will be increase in frequency. When the observer and the listener are moving away from each other then there will be decrease in the frequency.

Therefore, The Doppler effect is a change in the apparent frequency of sound caused by the motion of either their listener or the sound source.

the temperature and the pressure affect solubility. temperature affects by changing the properties of the solvent. the increase in pressure increases solubility, the decrease in pressure decreases solubility.