9. It still puzzles the science world why some mammals produce their young by________ (lay) eggs and hatching while others by______ (give) birth to young babies.

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閱讀下面材料，在空白處填人適當(dāng)?shù)膬?nèi)容（1個(gè)單詞）或括號(hào)內(nèi)單詞的正確形式。

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