Answer:Absolute Threshold
Explanation:
The sensitivity of any given sensory system to the relevant stimuli can be expressed as an absolute threshold
Maybe c or d
Step by step:
<h2>540 million years ago
</h2>
Paleozoic.
Era immediately follows Precambrian It lasted about 4.5 billion years.
When did abundant fossil evidence first appear in the geologic record.
540 million years ago.