<span>The answer is:
Penicillin was so heavily prescribed by doctors that it led to certain, more fit, strains of bacteria to be able to survive.
</span>
The pair of characteristics that one must use to classify a fish is the presence of jaw and skeleton type. Agnatha fishes are also called jawless fishes and they lack bony skeletons. Cartilaginous fishes are composed of cartilage skeletons. Bony fishes have bony skeleton structures.
Answer:
Echinoderms play numerous ecological roles. Sand dollars and sea cucumbers burrow into the sand, providing more oxygen at greater depths of the sea floor. This allows more organisms to live there.
Explanation:
They are also economically important to humans due to their use in scientific research and education and also for food.