根据短文，选择正确答案。
Mr. White looks out of his window. There is a boy at the there side of the street. The boy takes some bread out of a bag and begins eating it. There is a very thin dog in the street too.
The boy says to it, “Come here, good dog. I'll give you some bread.” The dog is hungry and goes to the boy, but he does not give it any bread. He kicks the dog. It runs away.
Then Mr. White comes out of his house and says to the boy, “I'll give you a shilling.”
The boy is happy and says,“Yes”. “Come here,” Mr. White says. The boy goes to him, but Mr. White doesn't give him a shilling. He hits him with a stick.
The boy cries and says, “Why do you hit me? I don't ask you for any money.” “No,” Mr. White says, “And the dog doesn't ask you for any bread, but you kick it.”
。则下列分子中的σ键是由一个原子的s轨道和另一个原子的p轨道以“头碰头”方式重叠构建而成的是