Given the e$-$commercial database schema shown below, write the following queries in $(1)$ relational algebra and $(2)$ SQL$.$ State your assumptions if the semantics are not clear.

item$($iid$,$ iname, price, category, rating, month, year$)$

customer$($cid$,$ cname, cstreet, ccity$)$

buy$($transID$,$ cid, iid, year, month, day$)$

advertise$($cid$,$iid$)$

$($a$)$ Find customers who have only bought items with a rating greater than $3.(12)$

$($b$)$ Find customers who have bought an item that was advertised to them. $(12)$

$($c$)$ Find customers who bought two of the same items in the same month.