Which of the Following Is Not a True Statement

Which of the following is not a true statement?

A) Absolute age is measured in the order by which events occurred
B) Relative age is based on the principle that an event can only impact a rock if the rock existed first
C) Absolute age measurement of rocks have become common since the second half of the 20th century
D) Prior to the second half of the 20th century, scientists relied upon relative age of rocks to estimate how old a rock layer was