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题目材料：
One proposed explanation for the Cambrian explosion, the relatively short evolutionary event during which most major animal phyla evolved hard external parts, is that the Cambrian Period was simply a hospitable time for animals to evolve. However, recent discoveries indicate that the eggs of Cambrian jellyfish and bristle worms are large compared with those of their ancestors. This and the close resemblance of the embryos to their adult forms are clues that they hatched fully equipped to enter the environment rather than passing through a series of juvenile stages. This strategy, known as direct development, is common under harsh environmental conditions but is not the usual method of development because a highly developed hatchling comes with a high energy cost to its parent.
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以上解析由 考满分老师提供。
