A: ①②③
B: ①②④
C: ①③④
D: ②③④
My Room
This is my room.It's nice and clean.You can see a ____ ,a ____ ,a ____ ,a ____ ,a ____ and a ____ in the room.There is a ____ on the wall. There are some ____ on the desk.There is a new ____ on the desk,too. I like my room.
用反证法证明命题“三角形三个内角至少有一个不大于60°”时,应假设( )