In the proof that vertically opposite angles are equal, two equations are written: ∠AOC+∠AOD=180∘ and ∠AOD+∠DOB=180∘. Which logical principle allows the deduction ∠AOC=∠DOB?
a) Common Notion: "If equals are added to equals, the wholes are equal"
b) Linear Pair Axiom
c) Common Notion: "If equals are subtracted from equals, the remainders are equal"
d) Supplementary Angles Theorem