The three critical concepts introduced alongside scarcity are marginal decision making, diminishing returns and:

1. The economic way of thinking is built from a small set of ideas. 2. The discussion of scarcity introduces three critical concepts. 3. Those concepts are opportunity cost, marginal decision making, and diminishing returns. 4. Elasticity and consumer surplus come later in the course. _Source: OpenStax Principles of Microeconomics for AP(R) Courses 2e (CC BY 4.0), Ch 2 'Choice in a World of Scarcity', sections 2.1-2.3_