challenge的形容詞形式是什么

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

　　挑戰(zhàn)媒體精英的不僅僅是讀者。

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

　　從前，實(shí)體書是一件具有挑戰(zhàn)性的東西。

　　Killy began challenging the basic theories of racing technique.

　　基利開始挑戰(zhàn)賽車技術(shù)的基本理論。