The single most important factor contributes to success is you do every single day．A. th

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A. that; that
B. which; what
C. which; what
D. that; what
解析：从句子结构分析,第一个空考查定语从句的关系词,因先行词前有形容词最高级修饰且指事物故只用that引导;第二个空跟在be动词后,表明是一个表语从句,用do后缺少宾语,因此只能由what来引导。