Question

which is a complete sentence?
two hydrogen atoms and one oxygen atom compose each molecule of water.
flea medicine for our cats is expensive, we order it online.

Explanation:

A complete sentence has a subject, a verb, and expresses a complete thought. The first option has a subject (Two hydrogen atoms and one oxygen atom), a verb (compose), and expresses a complete thought. The second option is a run - on sentence (two independent clauses "Flea medicine for our cats is expensive" and "we order it online" are wrongly joined by a comma instead of a semicolon or a conjunction).

Answer:

The first sentence "Two hydrogen atoms and one oxygen atom compose each molecule of water." is a complete sentence.