问题描述：
My sister swam for four years.She is ten now and she could swim .A.at the
My sister swam for four years.She is ten now and she could swim .A.at the age of four B.after she was six C.when she was six D.six years ag
My sister swam for four years.She is ten now and she could swim .A.at the age of four B.after she was six C.when she was six D.six years ag
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