Which of the following relational algebra queries computes the names of sailors who have reserved a red and a green boat?
a. πS.sname(σR.color='red'∧R.sid=S.sid)⨝σR.color='green'∧R.sid=S.sid)
b. πS.sname(σR.color='red'⨝R.sid=S.sid)∩πS.sname(σR.color='green'⨝R.sid=S.sid)
c. πS.sname(πR.sid(σR.color='red'⨝R.sid=S.sid)∩πR.sid(σR.color='green'⨝R.sid=S.sid))
d. πS.sname(σR.color='red'⨝R.sid=S.sid)∩πS.sname(σR.color='green'⨝R.sid=S.sid)