Be a pro. Back up your back up. Have at least one physical backup and one backup in the cloud. Have more than one of each. How much would you pay to retrieve all your data, photos, notes, if you lost them? Backups are cheap compared to regrets.
A book is a dream that you hold in your hand.
制造链式反应机器的基本点当然是确保每次裂变产生一定数量的中子,其中一些中子将再次产生裂变。