The numbers in the odd positions (1st, 3rd, 5th ,7th) are 3, 6, 9, 12. Each of these numbers is 3 more than the number before it. The numbers in the even positions (2nd, 4th, 6th, 8th) are 5, 10, 15, 20. Each of these numbers is 5 more than the number before it. Since the missing number is the gth position (odd), then the missing number is 12 + 3 = 15. 
What is the next number below?  3, 5, 6, 10, 9, 15, 12,20,~\uline{~~~~~~~~~~}~ 

(a) $$13$$ 
(b) $$15$$ 
(c) $$18$$ 
(d) $$25$$ 
(e) None of the above 