If you want to archive or backup your mails
1. Manual method
2. Online tool
3. Offline tool

No, in my opinion Bill Gates does not invest in Bitcoin but you can search the web for those people or companies who invest in Bitcoin.

Can anyone please tell me if Bitcoin is better than Conventional Currency. If yes, Why?

Yes, you can day trade bitcoin but it's not an easy work

Bitcoin was the first cryptocurrency to successfully record transactions on a secure, decentralized blockchain-based network. Each Bitcoin is basically a computer file which is stored in a 'digital wallet' app on a smartphone or computer

Covariance is a measure of the joint variability of two random variables. Covariance is used to obtain data. It measures the total variation of two random variables from their expected values.

Quantum Cryptography is the process of encrypting data or converting plain text into scrambled text so that only someone has the right key can read it.