Why is this layer called thermosphere?

Arrange the following layer of atmosphere vertically from the surface of the Earth.

1. Mesosphere
2. Troposphere
3. Stratosphere
4. Thermosphere

Represent the union of two sets by Venn diagram for each of the following.

$X=\{x | x$ is a prime number between $80$ and $100\}$

$Y=\{y | y$ is an odd number between $90$ and $100\}$