What is space charge in diode?

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 $100\}$

What are vacuum tubes?
 

What happens to the plate current in vacuum diode when the positive potential of the plate is increased?