challenge的形容词形式是什么

　　It is not just readers who are challenging the media elite.

　　挑战媒体精英的不仅仅是读者。

　　Once upon a time the physical book was a challenging thing.

　　从前，实体书是一件具有挑战性的东西。

　　Killy began challenging the basic theories of racing technique.

　　基利开始挑战赛车技术的基本理论。