Questions 18 and 20 are based on the following passage. At the end
of the passage, you will be given 15 seconds to answer the questions. Now,
listen to the passage. It can be inferred that ______.
A．all correctly printed stamps are Worthless B．Mauritius needed the stamps to send out invitations to a ball C．the printer was punished for his mistake D．collectors are constantly looking for stamps with mistakes