根据短文理解选择答案。
It is Sunday today. Ann is shopping with her mother. She wants her mother to buy a sweater for her.
In a clothes shop, she finds an orange one. She tries it on, but it's too small. She wants a bigger one, but the bigger ones are not orange. Ann doesn’t like other colours. Her mother asks, "Shall we go to another shop to have a look?" So they go out of this shop and go into another.
The second shop is much bigger than the first, and in it there are many kinds of sweaters of different sizes and colours. Ann tries on an orange one. It's too big. She tries a smaller one. It's OK.
“How much is it?” Ann’s mother asks the woman who sells clothes.
Then they find it too expensive, and they don't have so much money with them.
“Would you like a cheaper one?” The woman asks.
“No, we shall take this one. My daughter likes it. We shall come back to buy tomorrow.” Ann's mother answers.
After that, they leave the shop to buy some other things.
(硼氮苯)与
(苯)的分子结构相似，如下图
的同分异构体的数目为（ ）
