$18\div2=9$, $12-9=3$. 
There are twelve rooms in a building and each room has two windows and one light. Last evening, eighteen windows were lit. In how many rooms was the light off? (2016 Math Kangaroo Problem, Level 1-2, Question \#14) 
(A) $$2$$ 
(B) $$3$$ 
(C) $$4$$ 
(D) $$5$$ 
(E) $$6$$ 