Which two statements are true about a system?

A. A system is a group of objects analyzed as one unit.
B. Energy that moves across system boundaries is conserved.
C. There's only one way to define the boundaries of a system.
D. All systems are made by humans.

You want to move a heavy box with mass 30.0 kg across a carpeted floor. you pull hard on one of the edges of the box at an angle 30∘ above the horizontal with a force of magnitude 240 n, causing the box to move horizontally. the force of friction between the moving box and the floor has magnitude 41.5 n . what is the box's acceleration just after it begins to move