Sandy could do nothing but ______ to his teacher that he was wrong.

       A．admitting            B．admitted        C．admit       D．to admit

 Actually it was not until 4 June ______ the two foreign teachers arrived at our school.

   A. where            B. when           C. that              D. which

So many uses have been discovered for this wood ____ the supply of white pine is becoming smaller and smaller.

  A. then                     B. what                     C. that                       D. which

 Experts have been warning    of the health risks caused by passive smoking

   A. at a time     B. at one time    C. for some time   D. for the time