已知a是实数，$\frac{a+i}{1-i}$是纯虚数，则a等于（ ）

It's____(相当) hot. I want to swim with my brother.

A. When

B. Why

C. in

D. What

Mike: ____ is your favourite season ，Sarah?

Sarah: Summer.

Mike: ____ do you like summer?                               

Sarah: Because we wll have a party at home 0n Children's Day.       

Mike: ____is Children's Day?                                

Sarah: It's ____ Jane.

     Mr. Jackson is a teacher of science in a middle school. His students like his classes because his classes are very interesting. They can learn a lot in his class. The students can make（制作）many things. His little son Val also likes making things. He makes lots of machines（机器）, such as a machine like a man. It can do the job that is dangerous（危险的）for people to do. He says, “I want to be a scientist.”
根据短文内容，选择正确答案。