学习一门新语言会遇到很多挑战，但非常值得。

Learning a new language comes with many ___challenges, but it's very rewarding.

她向全班同学发起了数学竞赛的挑战。

She issued ___challenges to the whole class for a math competition.

科学家们挑战了这项研究的结论。

The scientists ___challenged the findings of the study.

他挑战我玩一局象棋。

He ___challenged me to a game of chess.