Although the causes of cancer _____, we do not yet have any practical way to prevent it.
| A．are being uncovered |
| B．have been uncovering |
| C．are uncovering |
| D．have uncovered |
Then professor walked onto the platform and seated himself in a chair, ______ for answering questions.
| A．had prepared | B．being prepared | C．preparing | D．prepared |
— Did you know any French before you arrived in Washington?
— Never ______ it, actually.
| A．had I learned | B．have I learned | C．I learned | D．was I learning |
She was such a proud person that she would die ______ she would admit she was wrong.
| A．rather than | B．until | C．after | D．before |
— Did your sister pass the exam?
— She failed and is in low spirits.
— I'm sorry for her.
— ______.
| A．Thank you | B．You're welcome | C．I would think so | D．Never mind |
He is only too ready to help others, seldom, ______, refusing them when they turn to him.
| A．if never | B．if ever | C．if not | D．if any |