3. [15pt] Consider the following relation schemas about a teaching database. Student: S(Sno,Sname Sgender, Sage,Sdept) Couse: C(Cno,Cname,Cprerequisite,Credit) Takes: T(Sno.Cno.Grade) Underlined attributes indicate the primary key. Attributes in italics represent foreign keys. Write the following queries as RA (relation algebra) expressions in the standard RA ndastion. (1) [5pt] Find the Sname of each student who took course with Cno's. (2) [5pt] Find the Sname of each student who did not take database. (3) [5pt] Find the Sname of each student who took operating system and database