For a, you would say that there is a lack in variation so any disease that affects one of them will affect them all. If there was a greater variation, some of them would have been resistant to the disease so some of them would not be affected by it, ensuring the survival of the species.
For b, it is probably something about selective breeding
For c, refer to this website:
http://www.cals.ncsu.edu/course/fw353/Estimate.htm