Who has a pen? I have ________.A.oneB.itC.thatD.so
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A解析one除作数词外还可作代词，替代可数名词。例：I lost my pen yesterday, I have to buy one now. （one相当于another pen）。试比较：Who has a pen? I have one.（one相当于a pen）．The picture is good. I'll take it. it代替上句提到的the picture。