It is a solid when is frozen and a liquid when it melts
"Changing water salinity" is the most significant challenge for organisms that live in estuaries.
<u>Answer:</u> Option D
<u>Explanation:</u>
For estuaries, alkalinity levels are usually the maximum at a river's mouth where the ocean water falls for, and the minimum upstream where freshwater falls in. Although salinity vary throughout the tidal cycle. In estuaries, salinity rates usually decrease in spring as snow melt and rain raises the freshwater flow from streams and groundwater.
It influences the chemical environments within the estuary, especially the dissolved oxygen (DO) levels in the water. The level of oxygen that would get dissolved in water or its solubility get declined when the alkalinity rises.
The perceived frequency when the fire truck is moving toward you and away from you will be 370 Hz and 329.59 Hz respectively.
<h3>What is the Doppler effect?</h3>
A sudden change in the frequency due to the distance between the objects and source is explained by the doppler effect.
As the source and observer travel toward each other, the frequency of sound, light, or other waves increases or decreases.
The perceived frequency when the fire truck is moving toward you;

The perceived frequency when the fire truck is moving away from you;

Hence, the perceived frequency when the fire truck in cases 1 and 2 will be 370 Hz and 329.59 Hz.
To learn more about the doppler effect refer to the link;
brainly.com/question/15318474
#SPJ1