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 black hole is formed by a high-mass star collapses. 
Fact 2: Fundamentally, a black hole is invisible.
answer:
a star collapsing can be invisible.


question:
Fact 1: Fractures heal when osteoclasts form new bone. 
Fact 2: Ligaments attach bones to bones.
answer:

fractures heal when osteoclasts form new body parts of the kind that ligaments attach to each other.