阅读下列材料，按要求完成各题。

    This is Mr. Wilson's room. It's clean and tidy. It looks like（像） a nice picture. The walls are white and the window is big. You can see a photo of his family and a photo of his girlfriend on the wall. His girlfriend is Peggy Carmen. A yellow desk is near the window. You can see some green plants and five colorful flowers on it. A blue clock is on it, too. It is seven now. A baseball is under his bed. His T-shirt and his bag are on the bed. Look! What's that in the drawer(抽屉) of the desk? Oh, it's Mimi, a cat. It's black. It looks like a hat.

已知：i．步骤②只有一种产物；C的结构中不含有甲基如图2

一个有理数的倒数是它本身，这个数是（       ）