Consider the following TypeScript code: 
Types image
Which of the following statements are correct:

A: The typescript compiler confirms that toUpperCase exists on the a instance.
B: The typescript compiler states that the toUpperCase function does not exist on the b instance.
C: The typescript compiler confirms that the toUpperCase function exists on the b instance.
D: RectangleType is an array where the type of a fixed number of elements is known.
E: Variables r, g, b are immutable.
F: r, g, b would compile to same type signature