Explanation
Step 1: Analyze the sets.
Set 1: (x−1)(x−2)=0 has solutions x=1 or x=2, which is a finite set.
Set 2: The prime numbers less than 199 are finite, so this is also a finite set.
Set 3: x5−1=0 implies x−1, which is a finite set.
Set 4: The set of odd numbers is infinite because there is no limit to how many odd numbers exist.
Step 2: Conclusion.
Thus, the infinite set is option 4, which consists of all odd numbers in N. Therefore, the correct answer is 4. {x:x∈N and x is odd}.
