How Has the Age of Mercury's Intercrater Plains Been Estimated

How has the age of Mercury's intercrater plains been estimated?

A) from residual radioactivity within the plains compared to areas outside, as measured by the Mariner 10 spacecraft
B) by comparing the albedo of the plains to areas outside because material becomes lighter as it gets older, as it does on the Moon
C) from radioactive dating of rocks returned to Earth by the Mariner 10 spacecraft
D) by comparing the number of craters per unit area within the plains to areas outside