If $\frac{1}{2}+\frac{2}{3}+\frac{3}{4}+\frac{n}{12}=2$, the value of $n$ is
(A) -4
(B) 13
(C) 18
(D) 4
(E) 1
Using a common denominator of 12 , we have $\frac{6}{12}+\frac{8}{12}+\frac{9}{12}+\frac{n}{12}=\frac{24}{12}$ or $\frac{23+n}{12}=\frac{24}{12}$.

Comparing numerators, we obtain $23+n=24$ or $n=1$.

ANSWER: (E)