Which of the following is a null set?

• A. {x : |x| < 1, x ∈ N}
• B. {x : |x| = 5, x ∈ N}
• C. {x : x2 + 2x + 1 = 0, x ∈ R}
• D. {x : x2 = 1, x ∈ Z}