F we apply an electrical stimulus to a muscle cell to cause it to contract, the magnitude of that stimulus must be strong enough to reach a critical value that is essential to initiate contraction. This critical value is known as .

critical value is known as threshold

Explanation:

solution

when we apply electrical stimulus to the cell of muscle

then magnitude of stimulus will be strong enough for reaching the critical value

and this critical value is threshold because when the size of an stimulus change in requirement for the detection, this is called the threshold

but if change is done by without stimulation then there threshold stimulus is absolute threshold

so correct answer is threshold

the answer is b, have negligible mass

54 degrees incident angle = reflected angle

angle is from normal turned to line of ray