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: Carbon may be stored in sedimentary rock for millions of years. 
Fact 2: Limestone is a rock composed of calcium, carbon and oxygen.
answer:
limestone contains carbon.


question:
Fact 1: a plant requires nutrients to grow. 
Fact 2: Legumes are typical of that type of plant.
answer:
legumes require nutrients to grow.


question:
Fact 1: Ancestors of hagfish are thought to have been the earliest vertebrates. 
Fact 2: Several agnathans, the lamprey and the hagfish, survive today.
answer:

Agnathans are thought to have been the earliest vertebrates.