The research is so designed that once ____ nothing can be done to change it.
| A．begins | B．having begun | C．beginning | D．begun |
His attitude to us is _____ of a comrade.
| A．one | B．it | C．that | D．those |
He was ill. _____ is why he didn't attend the meeting.
| A．This | B．It | C．There | D．That |
Will he come back to school tomorrow? I hope _____.
| A．that | B．this | C．so | D．it |
The students encouraged _____ to go on.
| A．one after another | B．one another | C．the other | D．one by one |
-- Who broke the glass?
- I suppose _____ must be one of the children.
| A．the one | B．he | C．it | D．who |