How many independent variables can you have in an experiment for it to be valid?

Jessica is traveling from miami, florida, to chicago, illinois. using the map, tell one way the land will change during the second half of her trip.

If an area has high air pressure and low humidity, what type of weather will it most likely have? plz !

In the experiment you asked to react hydrochloric acid and with sodium hydroxide. when measuring the volume of the reactants, which instrument would give the greatest precision.