All prime numbers are odd with the exception of two. Hence, if we consider the sum of the first million primes, it consists of one even number combined with 999,999 odd numbers. Since the product of an odd number multiplied by another odd number results in an odd number, we conclude that the sum must be an odd number, being even + odd = odd.
It's clear that an odd number ends with an odd digit, so the only digit that can be eliminated is b (an even digit).
