GATE CS 2025 Set 2 | Question 5

MCQ · 1 mark · −0.33

Let \(p_{1}\) and \(p_{2}\) denote two arbitrary prime numbers. Which one of the following statements is correct for all values of \(p_{1}\) and \(p_{2}\)?

  1. Option A: \(p_{1} + p_{2}\) is not a prime number.
  2. Option B: \(p_{1}p_{2}\) is not a prime number. (correct)
  3. Option C: \(p_{1} + p_{2} + 1\) is a prime number.
  4. Option D: \(p_{1}p_{2} + 1\) is a prime number.

Answer

(B)

Explanation

The product \(p_{1}p_{2}\) is divisible by \(p_{1}\), which is neither 1 nor the product itself (because \(p_{2} \ge 2\)). So \(p_{1}p_{2}\) always has a third divisor and is never prime, even when \(p_{1} = p_{2}\). Statement B holds for all primes.

One counterexample rules out each other option:

  • A: \(2 + 3 = 5\) is prime.
  • C: \(3 + 5 + 1 = 9\) is not prime.
  • D: \(3 \times 5 + 1 = 16\) is not prime.

Answer: B