City X has a population 3 times as great as the population of City Y, which has a population twice as great as the population of City Z. What is the ratio of the population of City X to the population of City Z? Choose one
A) 1 : 8 B) 1 : 4 C) 2 : 1 D) 6 : 1 E) 8 : 1

Rationale: "x = 3 y , y = 2 * z x : y , y : z 3 : 1 , 2 : 1 6 : 2 , 2 : 1 so , x : z = 6 : 1 ( d )"
Answer: D