When the population is divided into mutually exclusive sets,and then a simple random sample is drawn from each set,this is called:

A) simple random sampling.
B) stratified random sampling.
C) cluster sampling.
D) selection bias.

Before Christmas, the price of a necktie in a Patrick's Store for Men was $40. At the post-Christmas sale, the price was $26. By what percent had the price of the necktie been reduced?​