1, 3, 7, 15, 31, \cdots ..  2. 4. 8.~ 16.~ 32  31+32 = 64 
What is the missing number in the sequence below?  1, 3, 7, 15, 31,~\uline{~~~~~~~~~~}~ 
a. $$63$$ 
b. $$47$$ 
c. $$57$$ 
d. $$59$$ 
e. None of the above 