Complete parts $($a$)$ through $($c$)$ below.

a$)$ What is the population? Choose the correct answer below.

A$.$ All airline customers

B$.$ The $20,000$ frequent flyer customers of the airline

C$.$ All frequent flyer customers of the airline

D$.$ All frequent flyer customers

b$)$ What is the sampling frame? Choose the correct answer below.

A$.$ Frequent flyers of the airline in the database of all frequent flyers

B$.$ All frequent flyer customers

C$.$ The $20,000$ frequent flyer customers of the airline

D$.$ All airline customers