City x has a population of 3x10 to the fifth power. City y has a population of 6x10 to the sixth power. Which statement is true?
A. The population of city y is 2 times the population of city x
B. The population of city y is 20 times the population of city x
C. The population of city x is 300,000 less than the population of city y
D. The population of city x is 3,000,000 less than the population of city y