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: a valley is formed by a river flowing. 
Fact 2: Rivers erode the rocks they flow over.
[A]: Water erodes rocks as they flow over, creating a valley.


[Q]: Fact 1: Loss of ozone has created an ozone hole over Antarctica. 
Fact 2: ClO is the reactive form of chlorine that triggers ozone loss.
[A]: ClO triggers ozone holes.


[Q]: Fact 1: a rainbow is formed by refraction of light by separating light into all different colors. 
Fact 2: Rainbows are an optical phenomena due to the diffraction of light waves.
[A]:
a rainbow is formed by diffraction into all different colors.