Simplest Solution

A possible solution is shared in this archive

Note that

• in the Animal class, a EpsilonD constant was added to test if a double is “almost” 0,
• in the Bat class, a FlightSpeed property was added to facilitate testing the validity of its value,
• in the Bat class, objects are created even if the number of mammary glands is not 2 or 4, but a message is displayed,

This solution completes the “(easy) The ToString() methods return the information nicely formatted, as above.” bonus, and a solution to the “(medium) You explain briefly (in comment) why the SoundProduced method is not given as an abstract static method in the Mammal class.” can be as follows:

In short, static methods cannot be inherited or overridden in C#, and that is why they cannot be abstract. This is discussed in some details in this stack exchange post. A change was recently introduced in .NET 6, that makes static abstract members in interface possible, but it is limited to interfaces, and abstract static methods cannot be added to abstract classes.

Possible misconceptions

Some additional possible misconceptions are discussed in this project.