答案：A

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。