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: the emission of sulfur dioxide causes acid rain. 
Fact 2: Sulfur in the coal becomes sulfur dioxide when burned.
answer:
Burning coal causes acid rain.


question:
Fact 1: cutting down trees has a negative impact on an ecosystem. 
Fact 2: An ecosystem can be in a desert, a tropical jungle, an ocean or a pond.
answer:
cutting down trees has a negative impact on a jungle.


question:
Fact 1: Hermit crabs use the shells of dead snails for homes. 
Fact 2: Students live at home or in group homes.
answer:

Hermits crabs use dead snails shells as their place to live.