Which of the following statements is true about marginal revenue?
a) if marginal revenue is zero, it means that quantity demanded falls to zero when a firm changes its price.
b) if marginal revenue is negative, the additional revenue received from selling 1 more unit of the good is smaller than the revenue lost from receiving a lower price on all the units that could have been sold at the original price.
c) if marginal revenue is positive, the additional revenue received from selling 1 more unit of the good is smaller than the revenue lost from receiving a lower price on all the units that could have been sold at the original price.
d) marginal revenue increases as price falls and quantity sold increases.