Which of the following statements can be proved with a constructive existence proof (without using a computing device) ?
A. Every even integer can be expressed as the sum of two prime numbers.
B. There exists a prime number greater than 1000.
C. There is a positive integer solution to the equation x² + y² = 1.
D. For any real numbers a and b, there exists a real number c such that a + c = b.