challenge可作可数名词。
challengen.挑战；艰巨任务；挑战书；(比赛等的)邀请，提议；质询；质疑；提出异议
The role will be the biggest challenge of his acting career.
扮演这个角色将是他演艺生涯中最大的挑战。
She does not like anyone challenging her authority.
她不喜欢任何人挑战她的权威。
You can rely on him to respond to a challenge.
你可以信赖他，他懂得应付挑战。
