1. A type of backpack in which the frame is hidden inside the pack, reducing breakage by baggage handlers.
2. A frame of reference in which Newton's second law, F=ma, holds.
3. A uniformly running clock together with a measuring rod (or set of measuring rods) with uniform tick marks, which is not accelerating in any way.
4. A frame of reference in which Newton's third law holds for all types of force.

answer 1

answer 2

answer 3

answer 4

none of the above