Suggest an efficient design pattern for each of the design issues below and briefly explain how would you implement the suggested design pattern.$1.$ You are designing a library system and your design includes a class that represents the library inventory. You want to make sure that only a single instance of this inventory class is created.$2.$ The library inventory class consists of books $($holds a list of book objects$)$ and you want the other modules in your system to be able to traverse the list of books without exposing the data structure of this class.$3.$ You are designing an online system that offers multiple types of subscription modes $($such as free, pro and enterprise$)$ and your users change their subscription mode very frequently.