proof

保留你的收据作为购买证明。

Keep your receipt as ___proof of purchase.

孩子的微笑是动物园效用的最佳证明。

The smile of a child is the best ___proof of the utility of zoos.

她向我们展示了她的护照作为身份证明。

She showed us her passport as ___proof of her identity.