The average weight of students in a class is 50 kg. What is the number of students in the class?

A. The heaviest and the lightest members of the class weigh 60 kg and 40 kg respectively.

B. Exclusion of the heaviest and the lightest members from the class does not change the average weight of the students.

Using statement A, we cannot infer the number of students in the class.

Using statement B, we can say that the sum of weights of heaviest and the lighest persons is equal to twice the average weight of the class. But we cannot infer the number of students in the class.

Using both the statements as well, we do not have any information through which we can find the answer.

Hence, option D is the answer.