Which 02 sensor do I replace?

[elodam]

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?

[neon_rt]

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.