---Mr Ding,I have some trouble the passage!
----Remember it three or four times at least !
A,to understand ,reading B.understanding ,reading
C.understanding ,to read D.to understand ,to read
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解析：have trouble doing sth表示“在做某事方面有困难”是固定用法，故应该排除AD；remember后面既可以跟doing，也可以跟to do，当跟doing是，表示记得做过某事，而跟to do表示“记着做某事”本句的意思应该是“记着至少要读三四遍”，应该用“记着做某事”故用to read。所以本题选C。