A shell is fired from a cannon with a velocity $v$ at angle $\theta$ with horizontal. At the highest point, it explodes into two pieces of equal mass. One of the pieces retraces its path to the cannon. The speed of the other piece just after explosion is
(a) $3 v \cos \theta$
(b) $2 v \cos \theta$
(c) $\frac{3}{2} v \cos \theta$
(d) $\frac{\sqrt{3}}{2} v \cos \theta$