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: gills are used for breathing water by aquatic animals. 
Fact 2: Fish are backboned aquatic animals.
answer:
fish use gills for breathing water.


question:
Fact 1: changes in the pH of soil can cause plants to die. 
Fact 2: Sulfur lowers pH in soil.
answer:
Adding sulfer to soil can cause plants to die.


question:
Fact 1: tidal energy can be used to produce electricity. 
Fact 2: Tidal energy is one of the oldest forms of energy used by humans.
answer:

One of the oldest forms of energy used by humans can be used to produce electricity.