用反证法证明命题“在直角三角形中，至少有一个锐角不大于45°”，应先假设（　　）

完型填空
   When I was walking down the street the other day, I happened to1a small brown purse2 on the road. I3it up and opened it to see if I could4 the owner's name. There was5 inside it6 some changes (零钱) and an old photo--a picture of a woman and a young girl of about twelve years old who looked7 the woman's daughter. I put the photo back and8 the purse to police station, 9 I handed it to an officer. Before I left, the officer took my name and address10the owner might want to write and thank me.