The general rule is (

Magoosh):

If you do anything that “bunches the numbers together”, that decrease the standard deviation. If you do anything that “pulls the numbers further apart”, that increase the standard deviation.

And also:

In order to decrease the standard deviation of a set of numbers, you must add a value that is less than one standard deviation away from the mean

Given that standard deviation is positive (meaning, it is not equal to zero), then it means that adding 2 numbers with zero standard deviation will decrease the standard deviation of the set.

E) stand.dev of 6 and 6 is zero. It must decrease stand.dev