15. D。Do you know why? 用在文章最后用来引出原因，告诉读者其中的奥妙。
20
Peter’s job was to examine cars when they
crossed the frontier to make sure that they were not smuggling anything into
the country. Every evening he would see a factory worker coming __1__
the hill towards the frontier, __2__ a bike with a pile of goods of old
straw on it. When the bike __3__ the frontier, Peter would stop the man
and __4__ him take the straw off and untie it. Then he would examine the
straw very __5__ to see __6__ he could find anything, after which
he would look in all the man’s pockets __7__ he let him tie the straw
again. The man would then put it on his bike and go off down the hill with it.
Although Peter was always __8__ to find gold or other valuable things __9__
in the straw, he never found __10__. He was sure the man was __11__
something, but he was not __12__ to think out what it could be.
Then one evening, after he had looked __13__
the straw and emptied the worker’s pockets __14__ usual, he __15__
to him, “Listen, I know you are smuggling things __16__ this frontier.
Won’t you tell me what it is? I’m an old man, and today’s my last day on the __17__.
Tomorrow I’m going to __18__. I promise I shall not tell __19__
if you tell me what you’ve been smuggling.” The worker did not say anything for
__20__. Then he smiled, turned to Peter and said quietly, “Bikes.”