问题描述：
since then后面加什么时态?
Eg.
Scientists __________ (build) a station at the South Pole several yearsago.Since then they
________________ (get) muchinformation about the South Pole.怎么填?
Eg.
Scientists __________ (build) a station at the South Pole several yearsago.Since then they
________________ (get) muchinformation about the South Pole.怎么填?
问题解答：
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