Problem Solving

A set of 12 test scores has an average of 500 and a standard deviation of 50. Which of the following sets of additional test scores, when combined with the original set of 12 test scores, must result in a combined data set with a standard deviation less than 50?

(A) 6 test scores with an average of 450 and a standard deviation of 50.
(B) 6 test scores with an average of 500 and a standard deviation of 25.
(C) 6 test scores with an average of 550 and a standard deviation of 25.
(D) 12 test scores with an average of 450 and a standard deviation of 25.
(E) 2 test scores with an average of 550 and a standard deviation of 50.

Answer: B

Source: Manhattan GMAT