----- Is that Mr.Smith over there? ----- No, it _____ be him --- I am sure he is much taller.
The roommates have lived for three years and not once _____with each other.
My hometown, _____I spent my happy childhood, has changed a lot.
_____that the girl with an incurable disease would die soon, the boy decided to help her make her last days beautiful and meaningful.
Exploring _____space is exciting, yet dangerous. There is _____need for both robots and humans for space exploration.
Life is like time. Once _____, it never returns.