Which of the following didn’t occur during the cenezoic era

The formation of the ozone layer

answer; ///the correct answer is,by introducing a merit system of nobility.

- "bill clinton ( the 42nd u.s. president, served in office from by low unemployment, declining crime rates and a budget surplus. during clinton's second term, the u.s. economy was healthy, unemployment was low and the nation experienced a major technology boom and the rise of the internet."