【题目】________ the difficult maths problem, I have consulted Professor Russell several times.
A.Working out
B.Worked out
C.To work out
D.Work out

【答案】C
【解析】句意：为了解出那道数学难题，我已经向Russell教授咨询过好几次了。非谓语动词中只有不定式可作目的状语，题干中缺少目的状语，故选C。