Which of the following pairs of events are mutually exclusive? Justify your response.

a. {The next customer entering a retail store is older than 30 years.}, {The next customer entering the same retail store is younger than 20 years.}

b. {The next customer entering a retail store is younger than 20 years.}, {The next customer entering the same retail store will spend more than $900 in purchases.}

c. {The next customer entering the retail store will be older than 70 years and spend more than $900 in purchases.}, {The next customer entering the same retail store will try to steal an item.}