Given that $0 < a < b < c < d$, which of the following is the largest?
$\text{(A)}\  \frac{a+b}{c+d} \qquad\text{(B)}\ \frac{a+d}{b+c} \qquad\text{(C)}\  \frac{b+c}{a+d} \qquad\text{(D)}\  \frac{b+d}{a+c} \qquad\text{(E)}\ \frac{c+d}{a+b}$
Assuming that one of the above fractions is indeed always the largest, try plugging in $a=1, b=2, c=3, d=4$, since those are valid values for the variables given the constraints of the problem.  The options become:
$\text{(A)}\  \frac{1+2}{3+4} \qquad\text{(B)}\ \frac{1+4}{2+3} \qquad\text{(C)}\  \frac{2+3}{1+4} \qquad\text{(D)}\  \frac{2+4}{1+3} \qquad\text{(E)}\ \frac{3+4}{1+2}$
Simplified, the options are $\frac{3}{7}, 1, 1, \frac{3}{2}, \frac{7}{3}$, respectively.  Since $\frac{7}{3}$ is the only option that is greater than $2$, the answer is $\boxed{E}$.