A commutative ring $R$. The set $\Sigma$ consists of all ideals in which every element is a zero-divisor. Clearly $\Sigma$ has a maximal element $M$. I want to prove that $M$ is a prime ideal of $R$. (Atiyah & MacDonald Ex.1.14)
Here is my idea: suppose that $M$ is not prime. Choose $a, b\in R-M$ such that $ab=m\in M$, and argue that $(M\cup {a})$ is an ideal consisting of zero-divisors. But how to prove this? I have no idea.