Which 02 sensor do I replace?

I have a 1996 Dodge Stratus (2.4L Automatic). My check engine light keeps turning on and gives me a Lean State in Bank 1 code. The people at autozone told me it is the oxygen sensor. The only problem is there are two O2 sensors... How do I know which one to replace?

The one on the engine side of the CAT.
The one on the other side (downstream) of the CAT is for checking the function of the CAT.