我必须告诉你,出题人的英语知识不全面.known和discovered都是正确的选项,它们的后面都可以加补足语to get more help .又如：
1 We know him to be the best footballer here--------He is known to be the best footballer.
2 We discover(=find) that he is the best footballer here-------He is discovered(= found ) to be the best footballer
你可以参考------新世纪 新英汉词典 的349页-----上海译文出版社.