QUESTION $6$

Which of the following loops is best implemented with a while loop?

a$.$ Checking to see if a list of integers contains the value $12.$

b$.$ Counting how many keys in a dictionary start with the letter $\text{'}A\text{'}.$

c$.$ Asking the user to enter positive integers, exiting by entering $-1.$

d$.$ Looping through the characters in a string, and displaying 'yes' if it contains a vowel.