TASK DEFINITION: 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.
PROBLEM: Fact 1: when light enters the eye through the pupil , that light falls on the retina. 
Fact 2: Images are retained on the retina of a viewer's eye for a fraction of a second after they strike it.

SOLUTION: Light causes images to be retained on the retina.

PROBLEM: Fact 1: fluoride decreases tooth decay. 
Fact 2: Humans get most of their fluoride in drinking water.

SOLUTION: Drinking water decreases tooth decay.

PROBLEM: Fact 1: Fertilization occurs when sperm swim to an egg inside an archegonium. 
Fact 2: If the egg is fertilized, it can implant in the uterus and cause pregnancy.

SOLUTION:
pregnancy occurs when sperm swim to an egg inside an archegonium.