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: a valley is formed by a river flowing. 
Fact 2: Rivers cause erosion by transporting weathered rock and soil downstream.
answer:
Valleys are formed by erosion.


question:
Fact 1: water causes refraction of light. 
Fact 2: When light shines through a drop of water or prism, it bends or is refracted inside the droplet.
answer:
Light is bent inside of water droplets.


question:
Fact 1: Antibiotics have no effect on viruses. 
Fact 2: Where most viruses are DNA, HIV is an RNA virus.
answer:

antibiotics have no effect on HIV.