Select the number that will replace the question mark (?) in the given series.
1, 11, 20, 28, ?

1, 11, 20, 28, ?

Logic:  Differences between two consecutive terms is decreasing by 9, 8, 7,

11 - 1 = 10 

20 - 11 = 9

28 - 20 = 8 

Thus, correct option is (a).