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: Flagella help prokaryotes move. 
Fact 2: Some species swim by means of flagella Bacterial flagella are analogous to flagella of eukaryotes.

SOLUTION: prokaryotes and Bacteria move by swimming.

PROBLEM: Fact 1: Viruses cannot reproduce by themselves. 
Fact 2: Passwords multiply and reproduce amongst themselves.

SOLUTION: Viruses cannot multiply by themselves.

PROBLEM: Fact 1: erosion causes a river to become deeper and wider. 
Fact 2: Erosion is the movement of soil by water.

SOLUTION:
Water moving soil causes rivers to be deeper.