In a certain large forest, 25% of all the beech trees show damage by a certain parasitic fungus. A random sample of 20 beech trees are selected, then:
a) What is the probability that exactly half of the beech trees in the sample will be so damaged?
b) What is the probability that two or more beech trees in the sample will be so damaged?
c) What number of damaged beech trees will have the greatest probability of occuring?