The explicit statement in paragraph 4 that leaves no implicit suggestions is;
- Biologists backed up these ideas with sketches and models based on known animals.
An explicit statement is one that leaves nothing to the imagination. It states facts that are not open to interpretation.
The stated facts are clearly understood. In the third option, the author states a fact that is easily understood and that is not prone to interpretations.
It is just a clear fact that the Biologists backed up their ideas with sketches and models.
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Past perfect because of the combination of the auxiliary "had" and the past tense of the verb "fall", "fallen".
