Yesterday my sister and I went____(buy)something for her birthday．She didn't really know what____(get)but she seemed____(like)the idea of a pet，so we went to the nearest pet shop．She started ____(look)around at an the animals．The man in the shop advised her ____(choose)the rabbits，but when she saw some puppies(小狗) ____(play)in the box，she said she would like____(have)one of them．I didn't know if we had enough money____(buy)one and I hoped my mother would want us____(have)a dog in the house，but my sister promised____(look after)it well and we had enough money，so we bought a little brown dog.

①丁烷是由碳、氢两种元素组成的          ②丁烷由丁烷分子构成

③丁烷分子由碳、氢两种元素构成          ④丁烷由4个碳原子和10个氢原子组成

⑤丁烷分子由碳原子和氢原子构成

在体积为V L的密闭容器中通入a mol CO和b mol O₂，点燃充分反应后容器内碳原子数和氧原子数之比为（　　）