How can it be determined if an experiment is valid $($both internally and externally$)?$

The observed change in the dependent variable is in fact due to the experimental variable.

The results of the experiment apply to the general population outside the experimental setting.

The results do not account for the effects of extraneous variables.

Both the observed change in the dependent variable is due to the experimental variable, and the

results apply to the general population outside the experimental setting.

All of these are correct.