If $$1$$ more than a $$3$$-digit number is a $$4$$-digit number, then the numbers are $$999$$ and $$1000$$ and their sum is $$1999$$. 
Two whole numbers differ by $$1$$. If one number has $$3$$ digits and the other has $$4$$ digits, what is their sum? 
a: $$1001$$ 
b: $$1100$$ 
c: $$1999$$ 
d: $$2001$$ 