Paint.Net tool settings -- the Rectangular Select tool

This is the menu of options for the Rectangular-Select tool:

The red line and arrow show two of the options.

The underlined option has five choices: Replace, Add, Subtract, Intersect and Invert.

The drop-down menu designated by the arrow gives three choices: Normal, Fixed Ratio and Fixed Size.

Experiment with the different settings, then answer these questions:

1. What is the difference between the Normal and Fixed Ratio modes?

2. What is the difference between the Replace and Add modes?

3. What is the difference between the Replace and Subtract modes?