In each of 3—6, the relation R is an equivalence relation on A. As in Example 8.3.5, first find the specified equivalence classes. Then state the number of distinct equivalence classes for R and list them.
4. A = { a ,   b ,   c ,   d } R = { ( a ,   a ) ,   ( b ,   b ) ,   ( b ,   d ) ,   ( c ,   c ) ,   ( d , ​   b ) ,   ( d ,   d ) } equivalence classes: [a], [b], [c], [d]