北宋时期纸币出现的直接原因是（    ）

根据短文内容，选择正确答案。
   It’s Sunday morning．There are many people in the park，some boys  are playing basketball．There are some girls under a big tree．They’re singing and dancing．What are those women doing? They are drinking tea．Look at the woman in a blue coat．Who's she? She’s my mother．She is talking to Lin Feng．Lin Feng is her student．He is a good student．He studies hard．He’s good at all the subjects． And he's friendly to all his teachers and his friends．

《齐民要术》有一段描述：“凡五果，花盛时遭霜，则无子。天雨新晴，北风寒彻，是夜必霜。”据此完成下列各题。