Which one of the following statements is always true for any polynomial p(x) and real number a?
a) If p(a)=1, then x−a is a factor of p(x).
b) If p(a)=0, then x−a is a factor of p(x).
c) If p(a)=0, then x−a is a factor of p(x).
d) If p(a)=0, then x+a is a factor of p(x).