In a long-term study, the height of children at age 6 is recorded. For those same children, their height is again measured at age 16. In the linear regression model, which of the following statements are true?

1. The age 6 height is the explanatory variable.
2. The age 6 height is the response variable.
3. The age 16 height is the response variable.
4. Age 6 is on the vertical scale (y-axis) of a scatterplot.

Only A is true.

A and C are true.

Only C is true.

B and D are true.

C and D are true.