A、The card got damaged. B、The card was found invalid. C、The card reader broke down unexpectedly. D、The card reader failed to do

Behind the cash register at a store in downtown San Francisco,(16)Sam Azar swiped his credit card to pay for a pack of cigarettes. The store’s card reader failed to scan the card’s magnetic strip. Azar tried again and again. No luck. As customers began to queue, Mr. Azar reached beneath the counter for a black plastic bag.(17)He wrapped one layer of the plastic around the card and tried again. Success! The sale was completed. "I don’t know how it works, it just does. " said Mr. Azar who learned the trick from another clerk. Verifone, the company that makes the store’s card reader would not confirm or deny that the plastic bag trick worked.(18)But it’s one of many low-tech fixes for high-tech failures that people without engineering degrees have discovered, often out of desperation, and shared. " Today’s shaky economy is likely to produce many more such tricks. In postwar Japan, the economy wasn’t doing so great, so you couldn’t get everyday-use items like household cleaners. " says Lisa Katayama, author of Urawaza, a book named after the Japanese term for "clever lifestyle tips and tricks". "So people look for ways to do with what they had. "
    Today, Americans are finding their own tips and tricks for fixing malfunctioning devices with supplies as simple as paper and glue. Some, like Mr. Azar’s plastic bag are open to argument as to how they work or whether they really work at all. But many tech home remedies can be explained by a little science.
Questions 16 to 18 are based on the passage you have just heard.
16. What happened when Sam Azar swiped his credit card to pay for his purchase?
17. How did Sam Azar manage to complete the sale?
18. What is today’s shaky economy likely to do?