This table shows proportions of college freshmen in 1970 and in 2003 with various average grades in high school.
![1032_proportions of college freshmen.png](https://secure.tutorsglobe.com/CMSImages/1032_proportions of college freshmen.png)
a. Is the explanatory variable identified along the rows or the columns?
b. Discuss how the data provide evidence of "grade inflation."
c. In which one of the two years (1970 or 2003) were there students who were not accounted for in the four grade categories?