There are many married couples in which the husband and wife both carry a gene for cystic fibrosis but don’t have the disease themselves. Suppose we select one of these couples at random. According to the laws of genetics, the probability that their first child will develop cystic fibrosis is 0.25.

a. Interpret this probability as a long-run relative frequency.

b. If researchers randomly select 4 such couples, is one of these couples guaranteed to have a first child who develops cystic fibrosis? Explain your answer.