(A)  The sample space consists of 4 possible events

         (guessed 1st answer;         guessed 2nd answer)

         (guessed 1st answer;     not guessed 2nd answer)

         (not guessed 1st answer;     guessed 2nd answer)

         (not guessed 1st answer; not guessed 2nd answer)



(B)  This probability is   = .



(C)  This probability is   = .



(D)  "At least one answer correct" is the complement to "no one answer is correct".

      The probability "no one answer is correct" is what we computed in (B): this probability is  .


      Hence, the answer to (D) is 1 -  = .