A gastrovascular cavity is a digestive sac with only a single opening. An alimentary canal has two openings (a mouth and anus) between its digestive tubes unlike the gastrovascular cavity in which food is ingested and eliminated through the same opening.
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Explanation:
option D is the correct answer ..
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Irritation Threshold is defined as the name applies. It is the threshold or maximum point at which an individual has become irritated or annoyed.