which of the following statements is true in a reaction system at equilibrium? a. the number of collisions per unit time between reactants is equal to the number of collisions per unit time between products. b. the equilibrium constant is zero. c. reactants are reacting to form products at the same rate as products are reacting to form reactants. d. reactants and products are present in equimolar amounts. e. the product of the concentrations of the products divided by the product of the concentrations of the reactants is always a constant.