根据短文理解，选择正确答案。
A young woman was walking through a supermarket to pick up a few things when she noticed an old man following her around．Thinking nothing of it，she ignored him and went on shopping．After she got what she wanted，she went to checkout line，but the old man got in front of her．
“Pardon me!” he said，“I'm sorry if my staring（盯）at you has made you feel uncomfortable（不舒服的)，it's just because you look like my granddaughter，who just died recently．And I miss her very much．”
“I'm very sorry，”said the young woman．“Is there anything I can do for you?”
“Yes．” the old man said．“As I'm leaving，can you say ‘Good-bye，grandpa' to me? It would make me feel so much better．”
“Sure，”answered the young woman．
As the old man was leaving，she called out．“Bye-bye，grandpa!”
When she stepped(迈步) up to the checkout counter（收银台），she saw that her total was $178.50．
“How can that be?” she asked．“I only bought a few things!”
“Your grandpa said you would pay for him，” said the clerk.
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