Proof 01. 1 ⊢ A ∧ B ∧ C, hypo. 02. 1 ⊢ A ∧ B, conj_eliml 1. 03. 1 ⊢ B, conj_elimr 2. conj_elimlr. ⊢ A ∧ B ∧ C → B, subj_intro 3.