已知点A（1，3），B（4，﹣1），则与向量 $\overrightarrow{AB}$ 同方向的单位向量为（  ）

阅读理解

    Mary is a very little girl. She is only three years old. She can't read and write, but she likes drawing very much.

    One day, her sister, Jean sees her at the desk. She thinks Mary is drawing. She comes to her, but she finds Mary isn’t drawing. She is writing on the paper.

“What are you doing, Mary?” Jean asks her.

“I’m writing to Ann. ” She answers.

“But you don’t know how to write, how can you write to her?”

“Oh, it doesn’t matter(没关系). Ann can’t read and write, either(也). ”

欲建一座房子，使它的四周窗户都朝北，这座房子应建在（    ）