F is exact functor if and only if it transforms every exact sequence of A module

[mona123]

hi can someone please help me to solve this problem:

Let A and B be two rings
we consider the functor F:Mod_A→Mod_B
Show that F is exact if and only if it transforms every exact sequence of A module into an exact sequence of B module.

all what i know is this:

we say that by definition F is an exact functor if it transforms any short exact sequence of A module into short exact sequence of B module.

but i didn't know how to show the equivalence.
thanks in advance