p0131 code

ok i know this may sound stupid for asking this but i have a p0131 code coming up on my ecu i kinda went through a puddle and it happen. The code means the primary heated o2 sensor circuit low voltage.
heres the question is there only one o2 sensor on a 99 civic ex or are there 2? i know that there is one in front of the cat is that the only one?

nope theres 2. 2nd one should be after the cat

so with that code which one should i replace the one in front of the cat i dont remember seeing one after the cat.

P0131 Primary Heated Oxygen Sensor (Primary HO2S) (Sensor 1) Circuit Low Voltage
the first one