4. let r be the set of all real number r={(x1,xg ):x, e r, x, e r}. then, which one of the following is a subspace of r over r? (a) (2) x>0,x2 >0} (b) {x, x,) :

4. Let R be the set of all real number R={(X1,Xg ):X, E R, x, E R}. Then, which one of the following is a subspace of R over R? (a) (2) X>0,X2 >0} (b) {X, X,) :X, E R, x > 0} (c) {(X,X2} X<0,x <0} d) {(x, 0) :X E lR} number and e Rh and