How come I don't need to reference "System.dll" to use the "System" namespace?

I am working on an assignment that specified "Do not use any external libraries". So I created a c# application, and the first thing I did was remove all the dll's references by default... including "System.dll".

However, I can still add this to my code:

using System;
using System.IO;

I was just curious as to how come I do not need to have System.dll as a reference in my project to do this. Thanks!