**Step 1: Check what 91 is.**
91 is an odd number greater than 1, so I need to test divisibility by primes up to √91 ≈ 9.54 — that is, 2, 3, 5, and 7.
**Step 2: Test each prime.**
- **2:** 91 is odd → not divisible.
- **3:** digit sum = 9 + 1 = 10, which is not divisible by 3 → not divisible.
- **5:** 91 doesn't end in 0 or 5 → not divisible.
- **7:** 7 × 13 = 91 → **divisible!**
**Step 3: Conclusion.**
Since 91 = 7 × 13, it has divisors other than 1 and itself, so it is composite.
[No]