Consider the following TypeScript code:

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