Read the passage carefully and give the answer of question given below. Many doctors flatly refused to believe Jenner when he announced that he had found a preventive against smallpox. They declared vaccination to be a dangerous practice. But the dread of smallpox was in everybody's heart, and people flocked to Jenner to be vaccinated. The Latin word for cow is 'vacca', it is the root from which the word vaccination was formed, Some of the vacca' used by Jenner were not pure and some harms were done, but when supplies of pure vaccine were available, the practice of vaccinating spread all over England and from England to other countries. We hardly hear of outbreaks of smallpox now.
Vaccination sometimes proved harmful because

Options

A. vaccination was a dangerous practice
B. some of the vaccines used were of a poor hygienic standard
C. there are physiological differences between cows and human beings
D. vaccination is given at a very early age