If $a\otimes b = \dfrac{a + b}{a - b}$, then $(6\otimes 4)\otimes 3 =$
$\text{(A)}\ 4 \qquad \text{(B)}\ 13 \qquad \text{(C)}\ 15 \qquad \text{(D)}\ 30 \qquad \text{(E)}\ 72$
$6\otimes4=\frac{6+4}{6-4}=5$. 
$5\otimes3=\frac{5+3}{5-3}=4, \boxed{\text{A}}$