(9,12,15,18,21)

Which of the following pairs of numbers, when added to the set above, will increase the standard deviation of the set?

I. 14, 16

II. 9, 21

III. 15, 100

(A) II only

(B) III only

(C) I and II

(D) II and III

(E) I, II, and III

this question I couldnt find the answer because when I add every # to the set the same SD is there . Ex:

let me take the third :

the mean is 15

SD: 3

the new set when I add 15 is

( 24, 27,30,33,36 )

and

( 109,112,115,118,121 )

the SD is still 3

So , may anyone explain it for me and where is my main mistake ?