$20. \_ the difficult math problem, I have consulted Professor Russell several times.$
参考答案
C
_ the difficult math problem,…——2014 高考英语第 20 题答案解析
2014_省自主命题（陕西）
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2014_省自主命题（陕西）
$20. \_ the difficult math problem, I have consulted Professor Russell several times.$
C