Math Help - Radical of an ideal

$\sqrt{I}=\{ x \in A | \exists n such.that. x^n \in I \}$
B) Describe the radical of some ideal of $\mathbb{Z}$
C) If $A=K[X_1, \dots, X_n]$ prove that $\sqrt{I}\subset \mathcal{I}(\mathcal{V}(I))$