The correct answer is (d.) proteins. Higher organisms use nitrogen to make their protein. Food that is containing protein also involves nitrogen in it. Protein also has amino acids that contain nitrogen in it which is a necessary essential nutrient for the body.
It is false because the Doppler effect is something completely different <span />
Explanation:
Let the spring constant of two springs are k₁ and k₂. In series combination of springs, the equivalent of springs is given by :

So, if it is required to swap out a single spring for the two springs that are in series, the spring constant need to be to generate the same force at any displacement is,

Hence, this is the required solution.