Problems for addition and multiplication of probabilities. How can these problems be solved?

Question

Probability theory

1) Each of the two students in the test was given two questions. events:
Ai = {the first student correctly answered i-th question, i = 1; 2},
Bj = {The second student answered the j-th question correctly, j = 1; 2},
C = {only two correct answers were obtained}.
Express the event C through , Ai Bj
2) Reliability (probability of failure-free operation during time t) of the communication line between objects is 0.75. To improve the quality of communication, a backup line with a reliability of 0.65 was installed. Determine the reliability of communication between objects with a backup line.
Please help solve these puzzles. I already broke my head, I still don’t understand))) I would be very grateful)