The question below consists of a set of labeled sentences. Out of the four options given, select the most logical order of the sentences to form a coherent paragraph.

P. They are on the market already but they are costly.

Q. Their cost depends on the sort you install and the size of your home.

R. These devices extract warmth from the air or the ground, or from water - a bit like a fridge operating in reverse.

S. Climate advisers anticipate that most homes in future will be warmed by heat pumps.