Answer:
The answer is: measuring instrument
Explanation:
The researcher develops test to measure the variables she is interested in. So, they are her measuring instrument.
If there is air, then an object with greater mass falls toward Earth at a greater acceleration because it encounters less air resistance than if it had a smaller mass. There is no air resistance if there is no air, so they'll fall at the same acceleration.
A non-conservative force does NOT act upon the object