Which of the following demonstrates the law of inclusion-exclusion applied to find the probability that in two coin flips, at least one heads occurs?
a. P(at least one head) = P(head on first flip) + P(head on second flip) - P(head on both flips)
b. P(at least one head) = 1 - P(no heads on both flips)
c. P(at least one head) = P(head on first flip) * P(no head on second flip) + P(no head on first flip) * P(head on second flip)
d. P(at least one head) = 1 - P(both flips are tails)