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: Light enters the eye through an opening called the pupil. 
Fact 2: Light is essential to vision.
vision requires the pupil.

Fact 1: Sperm pass out of the body through the urethra. 
Fact 2: Urine leaves the bladder in a single tube, the urethra, that runs to the penis or to the vagina.
Sperm pass out of the body through a single tube that runs to the penis.

Fact 1: gases from burning coal and oil dissolving in water in the atmosphere causes acid rain. 
Fact 2: An acid is defined as a compound which can donate a proton .

A compound which can donate a proton from burning coal and oil dissolving in water in the atmosphere causes a compound which can donate a proton rain.