Question

heather is training to run a marathon at the end of the month. the more time she has spent training, the longer the distance she is able to cover during one run.
(t) = the amount of time heather has spent training
(d) = the distance heather is able to cover during one run
which variable is the independent variable?
(t)
(d)

Explanation:

An independent variable is the factor that is changed or controlled to observe its effect on another variable. Here, the training time ($t$) is the variable that influences the distance covered ($d$), so $t$ is the independent variable.

Answer:

$t$