“海宝”是2010年世博会的吉祥物，其形象如图所示．在点缀上海街头的各种“海宝”中，有一座“海宝”（材质均匀、实心）的质量为3.0×10³kg，密度为1.5×10³kg/m³，它的体积是多大？

He was told that it would be at least three more months _____ he could recover and return to work.

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A. when

B. before

C. since

D. that