Which economic concept is the closest to the saying "There's no free lunch"?

Suppose a monopoly pharmaceutical company produces a drug and sells 100 prescriptions for $25 each. In order to sell 101 prescriptions, the monopolist must lower the price to $24 per prescription. The marginal revenue of the 101st prescription is

A. -$76 B. $24 C. $25 D. $2,424

The demand for new capital depends on the interest rate.

Answer the following statement true (T) or false (F)