A. No. A "secret code" is a way of writing where you need to know a secret before you can understand what is written. For instance, "tfdsfu dpef" is written in secret code. It means "secret code", but every letter was replaced wit the next letter in the alphabet. Instead of writing "s", I wrote "t"; instead of writing "e", I wrote "f", and so on. B. "Serial number" usually refers to a product, while "reference number" usually refers to a transaction. For example, "Thank you for buying 20 of our TV sets. The *reference number* for your purchase is MPQ7. If you have questions about how much we charged you, please mention that *reference number*. The *serial number* for your TVs is 15678900654. If you have questions about how to use them, please mention that *serial number.*"