Gymnastics for the mind. Pair of puzzles

One task is to find a pattern, the second on logic.

Problem 1.

7 4 8
3 9 7
6 5 10
? 8 4

What should be the number instead of the "?"

Task 2. The
teacher asked his students to solve 4 examples. As a result, 3 out of 4 decided on two examples, but one did not solve one correctly.

Student Answers:
A: 50; 49; 51; 48.
B: 50; 48; 49; 51.
C: 48; fifty; 51; 49.
D: 51; 49; fifty; 48.

Determine the correct answers to the examples and indicate which of the students gave two correct answers, and which none.