$(8\%)$ For each of the following scenarios, give the "best" data structure or a combination of data structures, for example, an unsorted array, linked list, doubly linked list, circular linked list, stack, or queue. In each case, justify your answer briefly.

$($a$)$ Suppose that a shop decided that customers who come first will be served first. $(2\%)$

$($b$)$ A list must be maintained so that any element can be accessed randomly. $(2\%)$

$($c$)$ A program needs to remember operations it performed in opposite order. $(2\%)$

$($d$)$ The size of a file is unknown. The entries need to be entered as they come in$.$ Entries must be deleted when they are no longer needed. It is important that structure has flexible memory management. $(2\%)$