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。
J
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.”
