The veterinarians want to know the total for the invoices that have been paid.

a$.$ SELECT SUM$($INVOICE$)$

FROM APPOINTMENTS;

b$.$ SELECT SUM$($INVOICE$)$

FROM APPOINTMENTS

WHERE PAID $=\text{'}$Y$\text{'}$;

c$.$ SELECT SUM$($INVOICE$)$

FROM APPOINTMENTS

WHERE PAID $=\text{'}N\text{'}$;