The three types of collection differ in the way that an object is accessed.
- Set - references are effectively unsorted.
The set must be searched sequentially to access a specific object.
- Array - references are added with a numerical index value.
An object is accessed by specifying its index.
- Dictionary - references are sorted by one or more key values.
An object is accessed by specifying its key value(s).
You will use a Dictionary type of collection to store
the Book object references for a Borrower. | 
