Same start, two rules

Answer: C) 21. Each number is the sum of the two before it: 2 + 3 = 5, 3 + 5 = 8, 5 + 8 = 13, so next is 8 + 13 = 21. This is the famous Fibonacci sequence. 18 is the trap for repeating the last gap (5), and 26 for doubling the last number.

Answer: A) 13. These are the prime numbers in order: each one can only be divided exactly by 1 and by itself. After 11 comes 12, but that divides by 2; then 13, which is prime. So the answer is 13. 17 is prime too but it isn't next, and 15 is a trap: it is 3 × 5.