instruction:
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.
question:
Fact 1: Viruses cause many human diseases. 
Fact 2: Some smallpox viruses are maintained in laboratories.
answer:
smallpox causes human disease.


question:
Fact 1: when a plant grows , the size of the leaves of that plant may increase. 
Fact 2: Leaves are the main sites of photosynthesis.
answer:
when a plant grows the main site of photosynthesis increases in size.


question:
Fact 1: coal is used for generating heat by burning. 
Fact 2: Coal releases mercury when it is burned.
answer:

Some methods of generating heat release mercury.