The fact has worried many scientists______the earth is becoming warmer and warmer these years.
【正确答案】 C
【答案解析】分析句子结构可知，空处之后为同位语从句，所以应用在从句中不充当语法成分的that来引导，that引导的从句是对fact(事实，真相)具体内容的解释说明。故选C。