This field of scientific research remains________.
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.
The company gave away 300 dictionaries to the students at the beginning of the term, and _____500 the next term.