27．—Why is your professor so happy recently?
—Because the theory she insisted on ________correct．
| A．is proved | B．proving | C．was proved | D．has proved |
This is a very interesting book, I’ll buy it,______.
| A．how much may it cost | B．no matter how it may cost |
| C．however much it may cost | D．how may it cost |
It is difficult for us to learn a lesson in life_________we’ve actually had that lesson.
| A．until | B．after | C．since | D．when |
There were some chairs left over____ everyone had sat down.
| A．when | B．until | C．that | D．where |
The old man has a son and two daughters, ______treating him well, ____makes him sad.
| A．none of them; which | B．and none of whom; that |
| C．none of whom; which | D．and none of them; that |
—Will you tell the secret to the teacher?
—No!_________asked to.
| A．Even though | B．Until | C．Unless | D．When |