When defining an Invoice class, you can specify that its data fields will have private access so that

a$.$ other programs or methods can freely access or change those data fields

$$ b$.$ code outside of an Invoice object must use the object's methods to retrieve or manipulate its data

c$.$ a method that creates an Invoice object can directly assign values to its instance variables

$$ d$.$ multiple methods with the same name can act differently and appropriately with them