北宋前期，四川地区出现的世界上最早的纸币（   ）

根据短文理解，从A、B、C、D四个选项中选择最佳答案填空。

    Tom with his family 1 near the sea. Their house is not very high, but it is beautiful. There 2 big trees and a small garden in front 3 their house. They plant flowers in the garden. Some of the flowers are red, some are white 4 some are blue. Tom and his family love 5 garden very much.

      6 Tom and his sister go to school 7 bus. The bus is comfortable. There are air conditioners in the bus. And it is a double-decker bus. After school, they sometimes help their mum 8 some shopping at the shopping centre, and sometimes help their dad in the garden. At weekends, they often take some photos by the sea. 9 summer, they often go swimming 10 their parents.

用比值法定义的物理概念在物理学中占有相当大的比例，其中令你印象最深的采用比值法定义的两个物理公式是　____