A Uranium Nucleus (Mass 238 Units) at Rest Decays into a Helium

Multiple Choice

A uranium nucleus (mass 238 units) at rest decays into a helium nucleus (mass 4.0 units) and a thorium nucleus (mass 234 units) . If the speed of the helium nucleus is 1.8 * 10⁶ m/s, what is the speed of the thorium nucleus?

A) 1.0 *10⁴ m/s
B) 3.1 * 10⁴ m/s
C) 3.6* 10⁴ m/s
D) 4.1 *10⁴ m/s