In a large lecture course, the scores on the final examination followed the normal curve closely. The average score was 60 points and three fourths of the class scored between 50 and 70 points. The SD of the scores was
(i) large than 10 points
(i) smaller than 10 points
(iii) impossible to say with the information given.
Please prove why the answer is either (i), (ii) or (iii).