An Object of Mass M₁ Is Moving with a Speed

An object of mass M₁ is moving with a speed v on a straight, level, frictionless track when it collides with another mass, M_2, that is at rest on the track. After the collision, M₁ and M₂ stick together and move with a speed of

A) v.
B) M₁v.
C) M₁ + M₂) v/M_1.
D) M₁v/M₁ + M₂) .
E) M₁v/M_2.