Question

which is a complete sentence? you must be going somewhere nice your outfit is so elegant. two hydrogen atoms and one oxygen atom compose each molecule of water. we had an abundant harvest from our garden this year, we have plenty of squash and tomatoes.

Explanation:

A complete sentence has a subject, a predicate, and expresses a complete thought. The first option is a run - on sentence. The third option is also a run - on. The second option has a subject ("Two hydrogen atoms and one oxygen atom") and a predicate ("compose each molecule of water") and is grammatically correct.

Answer:

Two hydrogen atoms and one oxygen atom compose each molecule of water.