Scientists think that there has been life on Earth for years.
A. millions
B. millions of
C. million of
D. million
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解析：此题重点考查数词的表达法，如果有确切的数字 thousand 或million后面直接跟名词，thousand 和million等不用复数，也不用of，而thousands of，millions of直接跟名词或代词,前面不用确切的数字表示约数，根据题意故选B