Answer:
d. Penicillin may have played a role in the recovery of the four mice.
Step-by-step explanation:
In the experiment performed by Howard Florey and Ernst Chain, they used the mouse protection test for the search of potential antibiotics. For this purpose eight mice were injected with a lethal suspension of bacteria and four of these mice were also given penicillin through injection. They observed that the four mice that received penicillin as injection were lived and all the rest mice died. On the basis of the observation made in this experiment they concluded that penicillin worked as an antibiotic against serious bacterial infections and thus those mice which have been given penicillin were alive.