The correct answer is - C. Strains of bacteria can become immune to antibiotics that once killed them effectively.
One of the best examples where we can witness the evolution taking place are the bacteria. The bacteria can be very dangerous, so because of that people created antibiotics in order to control it or destroy it. But the bacteria has shown on multiple occasions that it actually is able to develop immunity to the antibiotics after few generations. Because of that, new antibiotics are constantly made in order to cope with the bacteria, but it constantly manages to strengthen its immune system, so its an ever going battle between the evolution of the bacteria and the scientists that try to kill it.
