Which of the following statements correctly elucidates the difference between an A.C. and a D.C. generator?
Statement I: An A.C. generator has commutator and a D.C. generator has slip rings.
Statement II: An A.C. generator has slip rings while a D.C. generator has commutator.
Statement III: A.C. generator rotates only in one direction. Statement IV: The polarity of the current changes for every half rotation in a D.C. generator.