阅读下面短文，写出括号中所给词的正确形式，使短文意思完整。

     Dolphins are 1_____ (interest) animals and researchers find them smart to watch. They 2_____ (have) no

hands, but can use tools to solve problems, just like us. There is no evidence that dolphins have a language of

their own, but they can understand many sign 3_____ (language). They can respond (反应) quite 4_____

(correct) and quickly. Dolphins live in a very different world, but they are good at 5_____ (do)our "intelligent

(智能的) tests”.