Which of the following statements about marginal utility is correct?
a. When marginal utility is negative, an increase in the quantity will increase total utility.
b. When total utility is increasing, marginal utility will be negative.
c. When marginal utility is positive, an increase in quantity will decrease total utility.
d. When marginal utility is zero, an increase in quantity will leave total utility unchanged.
e. When marginal utility is decreasing, an increase in quantity will decrease total utility.