从短文后每小题的三个选项中选出最佳选项。

    It is Sunday morning，Helen and her brother David are 1a walk in the park. There are lots of signs in the park. Helen knows a lot 2 them. But David 3. Suddenly Helen sees a boy climbing a tree. She says to the boy,"Don't climb the tree. Can't you see the 4Over there?" The boy jumps off the tree soon. David points to the sign and says to Helen，"So the sign means we 5climb trees，right?"  "Yes." says Helen.