Which one is bank 1 sensor 1? there are 4 total

Question

There are four sensors on my 2003 GMC yukon xl 5.3 motor.
Two before the cat. and two after. which one is causing the
p0135 code?

Answer 1

P0135 - 02 sensor heater circuit malfunction bank 1 sensor 1

As a rule of thumb bank 1 is where sparkplug number 1 is or the number 1 cylinder is, sensor 1 is the engine management sensor,