Triangle ABC is inscribed in a circle, and ∠B=∠C=4∠A. If B and C are adjacent vertices of a regular polygon of n sides inscribed in this circle, then n=
[asy]
draw(Circle((0,0), 5));
draw((0,5)--(3,-4)--(-3,-4)--cycle);
label("A", (0,5), N);
label("B", (-3,-4), SW);
label("C", (3,-4), SE);
dot((0,5));
dot((3,-4));
dot((-3,-4));
[/asy]
<spanclass=′latex−bold′>(A)</span>5<spanclass=′latex−bold′>(B)</span>7<spanclass=′latex−bold′>(C)</span>9<spanclass=′latex−bold′>(D)</span>15<spanclass=′latex−bold′>(E)</span>18