单选题
What is the mean of all the two-digit positive integers?
【正确答案】
C
【答案解析】The sum of all the numbers from I through n is 
. This means that the sum of all the numbers from I through 99 is 
and the sum of all the numbers from 1 to 9 is 
. This means that the sum of all the numbers from I through 99 is and the sum of all the numbers from 1 to 9 is