4. (10 points) An island contains two types of people: knight who always tell the truth and knaves who always lie. a) Two natives A and B approach you but only A speaks. A says: Both of us are knaves. What are A and B? b) You meet a group of six natives, U, V, W, X, Y, and Z. U says: None of us is a knight. V says: At least three of us are knights. W says: At most three of us are knights. X says: Exactly five of us are knights. Y says: Exactly two of us are knights. Z says: Exactly one of us is a knight. Which are knights and which are knaves