An Object of Mass $m$ Moving in A $+x$ Direction Collides Head-On with an Object at Rest with Mass

Multiple Choice

An object of mass $m$ moving in a $+x$ direction collides head-on with an object at rest with mass $3 m$ . After this elastic collision, the first object

A) Moves in the $+x$ direction.
B) Moves in the $-x$ direction.
C) Is at rest.
D) Could move in either the $+x$ or $-x$ direction depending on what its initial speed was.