Ten years ago, the sum of the ages of my mother and father was $$71$$. Each has aged $$10$$ years, so the sum of their ages is now $$91$$. 
Ten years ago, the sum of the ages of my mother and father was $$71$$. What is the sum of their ages today? 

a. $$51$$ 
b. $$61$$ 
c. $$81$$ 
d. $$91$$ 