我国地形复杂多样，五种基本地形类型中比重最大的是：（　　）

 -What are you doing?

   -I'm s____  the floor. It's dirty.

Don't copy others' work. If you do it by        , you'll feel much better.

—— It's said that scientists have ________ a way to deal with the disease(疾病)
——Really? That's great!