On a Survey,age Was Divided into Three Categories

Multiple Choice

On a survey,age was divided into three categories:
1) Younger than 18
2) 18 to 21
3) Older than 21
The researcher computed the mean age of her respondents by adding up the scores (the 1s,2s,and 3s) and then dividing by the number of cases.This was a mistake because the data are

A) only nominal and shouldn't be treated as ordinal.
B) continuous and shouldn't be treated as discrete.
C) interval-ratio.No mistake has been made.
D) ordinal and shouldn't be treated as interval-ratio.