instruction:
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.
question:
Fact 1: fossils are formed when layers of sediment cover the remains of organisms over time. 
Fact 2: All streams carry sediment .
answer:
Fossils are formed when layers of something streams carry cover the remains of organisms over time.


question:
Fact 1: Organs of excretion make up the excretory system. 
Fact 2: Leptospiral organisms cause infection in the kidneys and are excreted in urine.
answer:
Kidneys are part of the excretory system.


question:
Fact 1: Roundworms may be free-living or parasitic. 
Fact 2: Nematodes are microscopic, true roundworms.
answer:

nematodes can be parasitic.