They represent deadly bacteria<span> that have become </span>resistant<span> to </span>antibiotics<span> traditionally used to eradicate them
</span><span>Using antibiotics when ineffective or unnecessary enables the mutation and spread of antibiotic-resistant bacterial strains, since the bacteria will get small and/or a short duration of doses of the medicine, too short to kill the bacteria, but able to make them get used to the "poison" and no longer killed by it. That is why you are told by medical professionals that you should always take all antibiotics prescribed even when you start feeling better, because it takes the full amount of medicine to be sure the bacteria have been killed. You are also told not to skip doses for the same reason. </span>
The theme is the writer's view of or comment about the subject. The answer to your question is D. I hope this is the answer that you are looking for and it comes to your help.
Carbon dioxide is a greenhouse gas and traps heat in the atmosphere. Without it and other greenhouse gases, Earth would be a frozen world. But humans have burned so much fuel that there is about 30% more carbon dioxide in the air today than there was about 150 years ago, and Earth is becoming a warmer place.