15
9
17
11
Answer is Wrong!
Answer is Right!
The missing number is $\boxed{11}$.
The numbers in the sequence are all the prime numbers less than or equal to 20. The prime numbers are 2, 3, 5, 7, 11, 13, 17, 19. Therefore, the missing number is 11.
Option A, 15, is not a prime number. It is a composite number, which is a number that has more than two factors.
Option B, 9, is not a prime number. It is a composite
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