No code, please. Just explain. Thank you!

Question 2: Answer yes or no and Prove or explain precisely why 1. NPC problems are decidable 2. Problems that use an additional space of at most n2 are decidable. Hint: Say that y is written on the tape and the state is q. Also say that later this happens again: y is written on the tape the state is q, and the head in both cases points to the same character. This means an infinite loop. 3. Problems for which the number of possible solution is infinite are not decidable