问题描述：
英语38.Dr.Smith has ten more years before retirement.By then he ______ a cure for the disease.
38.Dr.Smith has ten more years before retirement.By then he ______ a cure for the disease.
A.has discovered B.must have discovered C.had discovered D.will have discovered
为什么
38.Dr.Smith has ten more years before retirement.By then he ______ a cure for the disease.
A.has discovered B.must have discovered C.had discovered D.will have discovered
为什么
问题解答：
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