Consider the following SQL schema:

create table Courses $($

id integer primary key,

code text,

title text,

quota integer check $($quota between $1$ and $9999),$

$...$

$)$;

create table Enrolments $($

student integer references Students$($id$),$

course integer references Courses$($id$),$

$...$

$)$;

Which of the following assertions captures the requirement that the total number of students enrolled in a course cannot be greater than the course quota?

$($a$)$

create assertion check$\_$quota

check $($not exists $($

select $*$

from Courses c join Enrolments e on $($e$.$course $=$ c$.$id$)$

group by e$.$student

having count$(*)>$ c$.$quota

$)$

$)$;

$($b$)$

create assertion check$\_$quota

check $($not exists $($

select $*$

from $($select c$.$id$,$ count$($student$)$ as nstu, c$.$quota

from Courses c join Enrolments e on $($e$.$course $=$ c$.$id$)$

group by c$.$id$,$ c$.$quota

$)$ as x

where x$.$nstu $>$ x$.$quota

$)$

$)$;

$($c$)$

create assertion check$\_$quota

check $($not exists $($

select $*$

from Courses c

where c$.$quota $>=($select count$(*)$ from Enrolments e where e$.$course $=$ c$.$id$)$

$)$

$)$;

$($d$)$

None of the above is correc