The following questions are based on four passages in English to test the comprehension of English language. Read each passage and answer the questions.
Read the following passage carefully and answer the questions that follow:
It was during one of the most dreadful small pox epidemics in England that Edward Jenner, a country doctor, made a discovery which was to alter the course of history. Jenner noticed that the disease seldom struck those who lived in a rural areas and worked around cattle. Most farmers and dairy workers had contracted cowpox and had recovered with nothing more serious than a pustule which left a scar. This observation led Dr. Jenner to think: why not vaccinate people with cowpox to protect them from smallpox? On May 14, 1876, Dr. Jenner took a healthy boy, James Philips, to a diary maid, Sarah Nelmes, who had a cowpox pustule on her hand resulting from an infection from her master’s cow. Dr. Jenner made two shallow cuts on James Phillips arm and inoculated them with matter taken from the cow pox sore. A pustule developed on the boy’s arm formed a scab and heated. In July of the same year. Dr. Jenner inoculated James with matter from a small pox pustule. During the next two weeks, the doctor watched for signs of small pox. They did not develop. The vaccination was successful. Dr. Jenner wrote a paper explaining his method of vaccination. At first the doctors were hostile and would not listen to a ridiculous procedure. In many towns people organized anti-vaccination campaigns. Gradually, however, the doctors and their patients accepted vaccination.
Dr. Jenner was successful as cowpox virus produces
A a mild infection in humans which is enough to produce active immunity
B no infection in humans
C severe infection in humans resulting in deaths
D a mild infection in humans which is not enough to
produce active immunity.