Help others whenever you can ______ you will make the world a nicer place to live in.
| A．and | B．or | C．unless | D．but |
While he was investigating ways to improve the telescope, Newton made ____ discovery which completely changed ________ man’s understanding of color.
| A．a ; / | B．a ; the | C．/ ; the | D．the ; a |
He never does anything wrong. He should be the ________ to blame.
| A．least | B．last | C．worst | D．best |
Reading provides the mind only with materials of knowledge; it is thinking that makes ________ we read ours.
| A．that | B．what | C．which | D．it |
We have done things we ought not to have done and ________ undone things we ought to have done.
| A．left | B．leaving | C．will leave | D．to leave |
I just don’t understand ________ that prevents so many children at school from being as happy as one might expect.
| A．what it is | B．what it does |
| C．what is it | D．why is it |