Answer: Option D
Step-by-step explanation:
The small pox virus would not be a good choice for a vaccine against cowpox. This is so because the intensity of the disease differs.
The small pox is more severe and its virus cannot be used in a attenuated form to generate immune response for small pox.
There are chances that doing so might kill the person with small pox as it has high fatality rate than cowpox. Even if its is cured it leaves marks and scars.
So, small pox virus cannot be used as a vaccine against cowpox.