Here is the passage:
Marina, the beautiful mermaid, wanted some
tuna salad. But had a small problem since she was allergic to celery. At
Sammy’s Sub Shop, Marina hoped to find tuna salad free of this dangerous
vegetable. Flopping across the tiled floor to the counter. Marina placed
her order and then checked her sandwich for celery. Not noticing, however,
the spoiled mayonnaise. At five o'clock that
evening, Marina became violently ill with food poisoning.
When a lifeguard at the beach discovered the problem, he called 911. Even
though the mermaid had fishy breath. A handsome paramedic gave her mouth-to-mouth
resuscitation. Wailing like a sick dog, the ambulance sped off to the hospital.
Where the doctor on call refused to treat a sea creature with a scaly tail.
A kind nurse, however, had more sympathy. When this caregiver returned with
a liquid antacid. Marina drank the entire bottle, feeling an immediate improvement.
The mermaid told the rude doctor never to swim in the ocean. For she would
order hungry sharks to bite off the doctor's legs. While sharp-clawed
crabs plucked out his eyes. Tossing her long hair, Marina thanked the nurse
for the antacid. And took a mint from David, the handsome paramedic.
A complete sentence must have a main clause. A main clause will follow this pattern:
Subject + Verb = Complete Thought.
In the highlighted item, Marina is the subject, and became is the linking verb. This pair makes a complete thought, so you do have a sentence.