It was not until the beginning of the twentieth century________to explore space.[ ]

Are you going to Shanghai— Yes, we Shanghainext Sunday.

The fact that she was foreign made__________difficultfor her to get a job in that country.[ ]

The water froze inside the pipe，causing it to ________and burst.

Look at the boy running on the playground. Is it David —- It ______ be him. I saw him go to the teachers’ office just now.