Which of the following scenarios would allow you to study the relationship between pressure and temperature of a gas according to the combined gas law without volume changing?

A balloon of gas heated with a hair dryer.
A closed thermos of air warmed a little in an oven.
Filling a 2-liter bottle half full of water then closing it and squeezing it.
Heating a thermos of air but allowing air to leak out to keep the pressure low.