An Old Remedy for Separating a Pair of Nested Drinking

An old remedy for separating a pair of nested drinking glasses stuck together is to run water at one temperature into the inner glass and then run water at a different temperature over the surface of the outer glass. Which water should be hot and which should be cold?

A) fill the inner glass with cold water while running hot water over the outer glass
B) fill the inner glass with hot water while running cold water over the outer glass
C) submerge both glass in a tub of hot water; wait for a while and then transfer to a tub of cold water
D) submerge both glass in a tub of cold water; wait for a while and then transfer to a tub of hot water