Which of the following rings are integral domains?<br>
1. The ring of integers<br>
2. The ring of Gaussian integers 
(that is, <i>Z</i>[<i>i</i> ] = {<i>a + bi </i>| <i>a, b</i> in <i>Z</i>}).<br>
3. The ring  <i>Z x Z </i><br> 
4. The ring <i>Z</i>[<i>x</i>]</i> of polynomials with integer coefficients. <br>
5. The ring <i>Z<sub>n</sub></i> of integers modulo <i>n</i>, <i>n</i> not prime<br>
6. The ring <i>Z<sub>p</sub></i> of integers modulo <i>p</i>, <i>p</i> is prime.<br>
7. The ring M<sub>2</sub>(<i>Z</I>)</i>, that is, 2 x 2 matrices of integers.