“想参加陪审团的公民按先后秩序依次进入，直到既定的人数到齐为止……开庭审理前，陪审员对案件一无所知，他们了解整个案情、进行判决的唯一依据是诉讼人的演说陈述。”古雅典的这一制度（   ）

如果单项式−$\frac{1}{2}$xay2与 $\frac{1}{3}$x3yb是同类项，那么a，b分别为（　　）

根据对话内容选择合适的答案填空。

A: Look, there are many animals in the zoo. Let's go and see them.

B: OK. Let's go.

A: Tom, there are so many birds in the tree.

B: Yes. There’s a monkey, too. I love monkeys.

A: There are two snakes under the chair.

B: Yes. But I don't like snakes.

A: Look, there are four chicks here.

B: How beautiful they are!

A: Two leopards are over there.

B: They are beautiful, too.