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: Viruses cause many human diseases. 
Fact 2: Some smallpox viruses are maintained in laboratories.
smallpox causes human disease.

Fact 1: Mollusks reproduce sexually. 
Fact 2: Some mollusks have a shell.
some animals with shells reproduce sexually.

Fact 1: when a metal rusts , that metal becomes orange on the surface. 
Fact 2: When iron oxidizes, it rusts.

When iron oxidizes, that metal becomes orange on the surface.