Select the combination of letters that when sequentially placed in the blanks of the given series will complete the series. s _ t m s r _ m _ r t _ s _ t m |
r, t, s, m, r r, t, s, m, r r, s, m, r, t r, s, t, r, m |
r, t, s, m, r |
s r t m / s r t m / s r t m / s r t m r , t , s , m , r |