AMC 8 1990 Problem 25

How many different patterns can be made by shading exactly two of the nine squares? Patterns that can be matched by flips and/or turns are not considered different. For example, the patterns shown below are not considered different.

$\text{(A)}\ 3 \text{(B)}\ 6 \text{(C)}\ 8 \text{(D)}\ 12 \text{(E)}\ 18$