A researcher initially plans to take an SRS of size n from a population that has mean 80 and standard deviation 20. If he were to double his sample size (to 2n), the standard deviation of the sampling distribution of the sample mean would be multiplied by

(a) !2.

(b) 1•!2.

(c) 2.

(d) 1/2.

(e) 1•!2n.