One fourth means (one half) of (one half) .
So the 18 months must be two half-lifes.
The half-life of this particular isotope is 9 months.
A difference between coelom and Pseudocoelom is that the gut of a pseudocoelom is surrounded by the endoderm, and the gut of a coelomate is surrounded by the ectoderm. So your answer is D.
Answer:
Species could go extinct when their environment changed if they could not adapt to the change.
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