1. a), b), c)
2. Generally, it is recommended but not required. It is required for certain situations for example, an when object of the class is going to be placed and searched inside a hash-based data structure
3. obj1 is less than obj2
4. No
5. StringBuilder
6. Allowing the implementation to change without changing the client code
7. More control over the code evolution and code flexibility for accommodating a change
8. More reliable code, quicker to write, less testing, easier for other people to understand
9. Other programmers that are going to maintain your code and you some time later
10. No, but it is very helpful to you