challenge的形容詞是什么

　　例句：

　　Bill was stimulated by the challenge.

　　比爾受到該挑戰的激勵。

　　She beat off a challenge to her leadership.

　　她戰勝了對她的領導地位的挑戰。

　　I hope that he proves equal to the challenge.

　　我希望他最后能應付這一挑戰。