Before any C3 teaching begins, the learner demonstrates verified-firm performance on the four U3 prerequisite atoms (U3.A1 force-as-magnitude+direction; U3.A4 gravity-toward-Earth; U3.A6 mass-as-inertial-obstacle; U3.A11 inertia-straight-line) plus C1 solar-system labels and orbit-as-path vocabulary, gating entry to the unit's plutonium program.