单选题 He (failed in) the competition, (that) (proved) that he was not working (hard enough).
【正确答案】 B
【答案解析】[解析] 应改为which。逗号后是非限制性定语从句，不能用that引导，应用which。句意：他在竞争中失败了，这一点证明了他工作不够努力。