—How amazing it is that astronauts are exploring outer space!
—It's a challenge, I guess, ______ man against nature.
| A．of | B．for |
| C．by | D．about |
Bob said he was going to join our club but he didn’t. He _______ his mind.
| A．can’t have changed | B．wouldn’t have changed |
| C．must have changed | D．should have changed |
Come and visit me whenever _______.
| A．you are convenient | B．you will be convenient |
| C．it is convenient for you | D．it will be convenient for you |
_______will Mr. Black be able to regain control of the company.
| A．Only by working hard | B．Only he works hard |
| C．With hard work | D．Only if work hard |
This type of computer is of great help and I’ll buy it _______.
| A．however may it cost | B．no matter how much it cost |
| C．however much it is cost | D．however much it may cost |
______ this new style of hat---maybe you will find it suitable for you.
| A．Having tried | B．Trying | C．Try | D．To try |