$$143-39=104$$,  $$104$$~$\div$ $$2$$ = $$52$$. 
The upper shelf of a bookshelf has $$143$$ books. The lower shelf has $$39$$ books. How many books should be taken from the upper shelf to the lower shelf so that both shelves will have the same number of books? 
a: $$52$$ 
b: $$104$$ 
c: $$91$$ 
d: $$46$$ 