---Look at the boy running on the playground. Is it David? ---It ______ be him. I saw him go to the teachers’ office just now.
Haze (雾霾)appears around us. Let’s do ________ helpful to protect the environment.
Nancy quickly ________ the door and put down her school things.
So many kinds of sweets! I really can’t decide _______.
Soon you _______ a senior high school student(高中生). Isn’t it exciting?
Please tell him to call me as soon as he _______ home.