Which of these is a sign of low protein in the diet of an animal?
A. Loss of appetite
B. Low birth weight of offspring
C. Low energy and fatigue
D. Swelling in muscles

I'd say all of the above. Heres some links hope it helps

https://www.webmd.com/diet/ss/slideshow-not-enough-protein-signs#:~:text=Weakness%20and%20Fatigue&text=And%20over%20time%2C%20a%20lack,oxygen%2C%20which%20makes%20you%20tired.

https://www.healthline.com/nutrition/protein-deficiency-symptoms

Explanation:

option a is correct i.e.

explanation:

in context to the membrane of nerve impulse, the outer membrane of neuron is semi permeable that is only sodium ions can pass through the membrane i.e. smaller ions can pass through the membrane while larger ions cannot pass through it.