Gymnastics for the mind. A couple of tasks

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

Task 1.

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

What number should be in place of the "?"
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Task 2.
The teacher asked his students to solve 4 examples. As a result, 3 out of 4 decided on two examples, and one did not correctly solve any.

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

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