我国跨境电子商务主要分为跨境___和跨境___两种模式。
A、B2B：B2C
B、零售；B2B贸易
C、交易；贸易

单项选择题Alchemists are people who ( ) A、create weapons for the country. B、produce food for people. C、create elixir(长生不老药) for kings or emperors. D、teach children at home.

多项选择题胸骨由（ ）三部分构成。