Astronomers Estimate That the Plains of Venus Are Only About

Astronomers estimate that the plains of Venus are only about 500 to 600 million years old. How do they estimate dates like this?

A) by looking for little brass plates that give the founding date and the name of the governor at the time
B) by looking at the radioactive rock samples that have been returned to Earth from Venus
C) by noting that all the plains on all the terrestrial planets have the same age
D) by seeing how much the continental plates on which the Venus plains are floating have spread apart
E) by counting the craters visible on the surface and comparing crater counts to other worlds