28 - 25 The arithmetic mean of ten different positive integers is 10. Then the maximum value of the largest number is 
(A) 10.
(B) 50.
(C) 55.
(D) 90.
(E) 91.
(7th American Junior High School Mathematics Examination, 1991)
[Solution] Suppose these ten numbers are all positive integers. Consider the extreme case: nine of the numbers are $1,2, \cdots, 9$, and the other number is 55. Their average value is 10.
Therefore, the answer is $(C)$.