While proving that vertically opposite angles are equal, we obtain
∠AOC+∠AOD=180∘ and ∠AOD+∠DOB=180∘. To conclude that ∠AOC=∠DOB, which of Euclid’s common notions is directly applied?
a) If equals are added to equals, the wholes are equal.
b) Things which coincide with one another are equal to one another.
c) If equals are subtracted from equals, the remainders are equal.
d) Things which are equal to the same thing are equal to one another.