设有关系R=（A，B，C）。与SQL语句SELECT DISTINCT A FROM R WHERE B=17等价的关系代数表达式是

A.∏A(R)

B.σB=17(R)

C.∏A(σB=17(R))

D.σB=17(∏A(R))

A.σR.C=S.C(πA,B,D(R×S))

B.πA,B,D(σR.C=S.C(R×S))

C.σ=((πA,B R)×(πDS))

D.σR.C=S.C(πD((πA,B（R）)×S）)

A.σR.C=S.C(πA,B,D(R×S))

B.πA,B,D(σR,C= S.C(R×S))

C.σR.C=S.C((πA,B (R))×(πD(S)))

D.σR,C=S.C(πD((πA,B(R))×S))

A.σR.C=S.C(πA,B,D(R×S))

B.πA,B,D(σR,C= S.C(R×S))

C.σR.C=S.C((πA,B (R))×(πD(S)))

D.σR,C=S.C(πD((πA,B(R))×S))

A.SELECT * FROM R，S WHERE R.C=S.C

B.SELECT A，B，D FROM R，S WHERE R.C=S.C

C.SELECT A，B，C FROM R，S WHERE R=S

D.SELECT A，B ,C FROM R WHERE C IN (SELECT C FROM S )