Which of the following subsets of p2 are subspaces of p2? (p2 denotes the set of polynomials of degree 2 or less.) a. {p(t) | p′(2)=p(4)}{p(t) | p′(2)=p(4)} b. {p(t) | p(8)=2}{p(t) | p(8)=2} c. {p(t) | p′(t){p(t) | p′(t) is constant }} d. {p(t) | p′(t)+9p(t)+6=0}{p(t) | p′(t)+9p(t)+6=0} e. {p(t) | p(6)=0}{p(t) | p(6)=0} f. {p(t) | ∫30p(t)dt=0}{p(t) | ∫03p(t)dt=0}