Nancy Lerner is taking a course from Professor Goodheart who will count only her best midterm grade and from Professor Stern who will count only her worst midterm grade. In one of her classes, Nancy has scores of 30 on her first midterm and 50 on her second midterm. When the first midterm score is measured on the horizontal axis and her second midterm score on the vertical, her indifference curve has a slope of zero at the point (30, 50). Therefore this class could

A. be Professor Goodheart's but could not be Professor Stern's.

B. be Professor Stern's but could not be Professor Goodheart's.

C. not be either Professor Goodheart's or Professor Stern's.

D. be either Professor Goodheart's or Professor Stern's.