-7的相反数是（）

Dear Liz,

    Thank you very much ____ your letter and the photos of your ____ (friend) and family. Your parents and friends look really nice ____ the photo. Now I can tell you something about me and ____ (I) family here in New York.

    I've got two brothers. ____ (they) names are Lukas and Andreas and they ____ (be) 16 and 19. My mother is French and her name is Christine. Dad is American and his name's Dieter. Our house ____ (have) got four bedrooms and a small garden. My room is tidy, but my ____ (brother) is not. They don't often clean it. We have a dog, Zak, and we think he's cute. We ____ (love) it a lot.

    Please tell me all about ____ (you) friends and school in your next letter. I'd like to know about English schools.

Love,

Laura

牙科医生用来观察病人牙齿的小镜子，要放在火上烤一下才放进病人的口腔中，医生这样做是为了（　　）