What geological principle states that rocks at the bottom of a sequence are older than the rocks above?

Two friends at different locations want to communicate with each other by sending low energy signals. which of the following methods can they use to communicate? a) produce x-rays using colliding electrons and send them to radios, which capture sound b) send messages using infrared radiation, which travel in the form of waves c) send radio waves through intervening media like radio and television d) produce sound waves using microwaves from heated objects

Which of these is an intensive property of a substance

Which scientists contributed to the determination of how cfcs in clouds in the upper atmosphere could destroy ozone molecules