若命题 $p: \exists x \in[-3,3], x^2+2 x+1 \leq 0$, 则命题 $p$ 的否定是
A. $\forall x \in[-3,3], x^2+2 x+1>0$
B. $\forall x \in(-\infty,-3) \mathrm{U}(3,+\infty), x^2+2 x+1>0$
C. $\exists x \in(-\infty,-3) \mathrm{U}(3,+\infty), x^2+2 x+1 \leq 0$
D. $\exists x \in[-3,3], x^2+2 x+1 < 0$