Two objects are moving to the right. Object $1$ catches up with object $2$ and collides with it$.$ The objects stick

together and continue with velocity vf$.$ Which of these statements is true?

A$.$ vf $=$ v$1$

B$.$ vf $<$ v$2$

C$.$ vf $>$ v$1$

D$.$ v$1>$ vf $>$ v$2$

E$.$ It is not possible to decide without knowing the masses.