Which muscle characteristic describes the ability of muscle to respond to a stimulus?

hiya!

"Excitability is the ability to respond to a stimulus, which may be delivered from a motor neuron or a hormone. Extensibility is the ability of a muscle to be stretched."

complete the square on the right side of the equation;

-5(x + 0)^2 + 15

answer: d. y = -x^2 + 15

b.have there own function.