64. Go back ___ you came from.    A. until       B. where       C. which        D. when

63. The doctor will not perform the operation ___ it is absolutely necessary.

A. when              B. if                C. for                D. unless

62. Difficulties are nothing ___ we are not afraid of them.

A. for                B. as                C. if                D. whether

61. I'd like to go swimming ____ the water is not too cold.

A. for               B. unless             C. if                D. whether

60. I can't use your pen, ___ there is no ink in it.        A. for    B. when   C. if    D. whether 

59. Things have changed a lot ____ I wrote to you last time.

A. when              B. since         C. as         D. before

58. ___ he came to study in the university, he has made much progress in the study of English.

A. While            B. When             C. Since             D. After

57. I'll come and see you _____ I go to the countryside.

A. while             B. when          .   C. as soon as         D. before

56. Don't try to get off the bus ____ it has stopped.

A. while             B. as                C. since             D. before

55. He will tell you about it ____ you get there.        

A. while      B. as      C. when      D. /