Which of the following uses for indexes can be expected to improve performance? $($Select all uses that would normally be expected to improve performance of requests.$)$

a$.$ Indexing columns that are of high selectivity, such as Social Security Number, when those columns appear in WHERE clauses.

b$.$ Indexing columns that are not referenced in WHERE clauses.

c$.$ Indexing the foreign key columns of tables that reference other tables and are frequently joined to the other tables using an equality

d$.$ Indexing columns with long data types such as CHAR$(1024)$

e$.$ Indexing columns that appear frequently in WHERE clauses but are not primary keys.