15. C。本句的主语是they，而each只是同位语，所以不可用has，have一般不用进行时态，所以不可选D。
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It was too late
at night when an old man came to a small town. He found an inn(小旅馆) and wanted to stay there for
the night. After he 1 his room, the owner said to his wife,
“Look at his bag. There 2 much money in it. Let’s 3 when he’s asleep, 4 ?”
“No,
no,” said the woman. “He must look 5 his bag tomorrow morning. If he can’t
find it, he’ll telephone the police.”
They
thought for 6 minutes. Then the woman had an idea. “We
have forgetful grass. Why 7 some forgetful grass into his food? If he
8 the food, he will forget
9 his bag away.
The
old man had the food 10 the forgetful grass and went to bed. The
next morning when the owner got up, he found the door 11 and the old man had left with the bag. He
woke his wife up and said to her 12 , “What a fool(傻瓜)! You forgetful grass isn’t 13 at all.”
“No,
I don’t think so. He must forget 14 ,” his wife said.
“Oh!
I remember now!” cried out the man suddenly. “He forgot to 15
for the night.”