An airline company is interested in the opinions of their frequent flyer customers about their proposed new routes. specifically, they want to know what proportion of them plan to use one of their new hubs in the next 99 months. of their customers, 45% are silver-level, 30% are blue, and 25% are red. they first compile separate lists of silver, blue, and red members and then randomly select 4500 silver members, 3000 blue members, and 2500 red members to e-mail. they take that sample of 10,000 and send them an e-mail message with a request to fill out a survey in exchange for 2000 miles. what kind of sampling method have they used?