题目内容：
Let me tell you a story about Bert and Mildred Bumbridge, who used to be very forgetful. For example, Mildred would forget to cook dinner, or Bert would show up for work on Sunday thinking it was Monday. One summer, they were to take a long plane trip. What do you suppose happened? Well, they got to the airport with only ten minutes to spare. In that situation, anyone would board the plane right away. But not'Mr, and Mrs. Bumbridge. They just had to buy some flight insurance first. After all, who knows what will happen on a plane flight? They quickly put some coins into a machine and out came their insurance policy. “Who would get the money if we crash, I wonder?”asked Mildred. “My mother, of course,”her husband replied. “We'll mail the policy to her. Now quick, give me a stamp ,will you?”he said. “The plane is going to take off in another minute.”Bert put the stamp on the envelope, dropped it in the mail box, and suddenly let out a cry. What happened, do you suppose? He had mailed their plane tickets to his mother!根据以上内容,回答题.
What did Bert and Mildred get after putting coins into the machine? A．A stamp.
B．An envelope.
C．Two tickets.
D．Some flight insurance.
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