Researchers Determine That Age Has a Positive Correlation with Trust

Researchers determine that age has a positive correlation with trust of one's neighbours. They then ask whether this correlation is due to the fact that older people tend to have lived for longer amounts of time in their neighbourhood relative to younger people. What type of variable is "time lived in neighbourhood" for this example?

A) a beta variable
B) a zero-order variable
C) a dependent variable
D) a control variable