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.

Fact 1: rain causes erosion. 
Fact 2: Erosion is the breaking apart and moving of rocks and soil.
Rain causes the breaking apart and moving of rocks and soil.

Fact 1: Monotremes are mammals that reproduce by laying eggs. 
Fact 2: Like the platypus, an echidna is an egg laying mammal.
Platypus are monotremes.

Fact 1: air pollution can cause the pH of soil to decrease. 
Fact 2: Acidity is measured with a pH meter or with pH paper strips.

air pollution can cause the Acidity of soil to decrease.