I hope the little I have been able to do has been of some use.A. that; thatB. 不填；by whic

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(答案→)A. that; that
B. 不填；by which
C. what; what
D. 不填；with which
解析：考查宾语从句和定语从句。     the little      I have been able to do has been of some use是一个宾语从句，从句不缺成分，用that引导，也可省略。     I have been able to do是定语从句，先行词是the little，在定从中作do的宾语，用that引导。所以选A。