Alex goes to the store to buy snacks. While he's there, he remembers he also has run out of deodorant. When Alex gets to the cashier, he finds that he's forgotten his wallet and has only $5 in cash to spend. Alex has to choose which items to buy. Which of the following key ideas of economics does this illustrate?

a. marginal analysis
b. market failure
c. tradeoffs
d. incentives