Practicing Success
Select the combination of letters that when sequentially placed in the blanks of the given letter series will complete the series. T _ R _ _ S _ D T _ R D _ S _ D |
S T S R R T R S D T D S T T D S T R T T R S D T R S T R |
S D T R S T R |
T S R D / T S R D / T S R D / T S R D S D T R S T R |