Question 1
Which subset of $\mathbb{Z}$ is a subring under the usual addition and multiplication?
Question 2
If $S$ and $T$ are subrings of a ring $R$, which set is always a subring of $R$?
Question 3
To prove that a nonempty subset $S$ of a ring $R$ is a subring, which condition is enough?
Question 4
Which statement is always true about the zero subring?
Question 5
Which set is a subring of $\mathbb{R}$?
