The primary neurotransmitter at the neuromuscular junction is

acetylcholine

Explanation:

Acetylcholine is the neurotransmitter used by the motor neurons to stimulate action potentials in the muscle cells

Explanation:

Newton's 2nd Law say that F = ma which is the sum of the forces acting on an object is equal to the object's mass times the acceleration at which it is moving. That means that the forces acting on this tree are F which is the Force the man needs to exert to move the tree, and f which is the force the tree is exerting in an effort to stay right where it is. That alters our 2nd Law equation to F - f = ma. BUT if the tree isn't moving, the acceleration is 0, making the whole right side of that equation go to 0. That means that F = f. Phew! Let's do that.

f = μ where f is the frictional force, μ is the coefficient of static friction, and is the weight of the object, which are given as 670 N. Filling in:

f = .5(670) so

f = 335 N (that is not using the rules of significant digits properly, but if you use them properly, the answer is not one of the choices, so...)

C is the one you want.