One hundred teachers attended a seminar on mathematical problem solving. The attitudes of a representative sample of 12 of the teachers were measured before and after the seminar. A positive number for change in attitude indicates that a teacher's attitude toward math became more positive. The 12 change scores are as follows: 3; 8; -1; 2; 0; 5; -3; 1; -1; 6; 5; -2 What is the standard deviation for this population? O a. -3.50 O b.-.25 O c. +3.50 O d. +.25