A vehicle's oil pressure warning light is on. Technician A says that the pressure sending unit should be replaced and the vehicle re-checked. Technician B says that a mechanical oil pressure gauge should be used to check the oil pressure before replacing the sending unit. Which technician is correct?

Technician B is correct

Explanation:

Unless the car owner just put oil in the car, the sudden distrust of the pressure sending unit is alarming.

Even if the car owner just replaced oil, so many other things can cause a running out of oil.

So, the better option is to check the oil pressure with a gauge outside of the car system before deciding to replace the pressure sending unit.

Nope.  Just the opposite.

That's why, when you open a new bottle of soda and
let the pressure inside come down to atmospheric
pressure, some of the sparkly gas comes out of solution
and goes "hisss" out of the bottle.  (If you open the bottle
too fast, then some of the soda comes out with the gas.)