Mark all correct statements regarding query execution

$($a$)$ It is preferable to have indices available for those join operations that are

executed often.

$($b$)$ The predicate$-$pushdown strategy "pushes down" a selection operation to be

executed at the end of an execution plan.

$($c$)$ An existing index can sometimes boost the speed of a join operation.

$($d$)$ Materialized views can speed up insert, update, and delete operations but

penalize look$-$ups.

$($e$)$ It is preferable to create indices on frequently joined attributes.

$($f$)$ During database design, it is optimal to set up foreign key attributes for which

we know a primary or clustered index will be created.