他的行为被视为对总统权威的直接挑战。

His actions were seen as a direct ___challenge to the authority of the President.

杰克显得孩子气的唯一一次是当他挑战拉尔夫领导地位失败时，他羞愧地哭泣。

The one occasion in which Jack seems childish is when his ___challenge to Ralph's leadership fails and he weeps with humiliation.

新证据对现有法律框架提出了严重挑战。

The new evidence presented a serious ___challenge to the existing legal framework.