【正确答案】 C

【答案解析】[解析] 画线词deadly意为“致命的”。A项contagious意为“传染性的”；例：Chickenpox is a contagious disease. 水痘是一种传染病。B项serious意为“严重的”；例：Doctors said that his conditions were serious but stable. 医生说他的情况严重但还稳定。C项fatal意为“致命的”；例：There was a fatal flaw in the plan. 计划中有一个致命的缺陷。D项worrying意为“令人担心的”；例：His conditions are worrying to us. 他的情况令我们担忧。故选C。