Combine the given two facts to write a concluding fact. Note that there should be some parts of the first and second facts that are not mentioned in this conclusion fact. Your combined fact should be the result of a chain between the two facts. Chains form when two facts connect together to produce a concluding fact. An example of a chain is: "pesticides cause pollution" (fact1) + "pollution can harm animals" (fact2) → "pesticides can harm animals" (conclusion fact). Parts of the concluding fact overlap with the first and the second fact. To construct such a  concluding fact, a useful way is to borrow the subject from one fact and the ending conclusions from another fact.

[Q]: Fact 1: energy is used for heating a building by a heating system. 
Fact 2: Energy used for heating is the single most commonly used form of energy worldwide.
[A]: Heating buildings is a very common production of energy worldwide.


[Q]: Fact 1: fluoride decreases tooth decay. 
Fact 2: Humans get most of their fluoride in drinking water.
[A]: Drinking water decreases tooth decay.


[Q]: Fact 1: Some parasites kill their host, but most do not. 
Fact 2: All parasites are harmful to their host, but some are beneficial to humans.
[A]:
Some parasites are beneficial to humans.