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: Echinoderms use pheromones to communicate with each other. 
Fact 2: Echinoderms are represented by the starfish.

SOLUTION: Starfish use pheromones to communicate with each other.

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: darkness has a negative impact on photosynthesis. 
Fact 2: Carbohydrates in plants are formed by photosynthesis.

SOLUTION:
Darkness has a negative impact on the amount of carbohydrates formed in a plant.