Newton's third law of motion states that if a force is exerted on an object, another force occurs that . A.

is in the same direction and speed
B.

is in the same direction and size
C.

is equal in speed and opposite in direction
D.

is equal and opposite in direction

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explanation:

the box does not move because you need to use a force greater than 300 n. you are only using 50 n. it needs to at least be 301 n for the box to move because then it is an unbalanced force and it will move the way that the greater force is pushing it. if you use only 300 n it will be a balanced force and will not move because the force you are exerting on the   box is the same as its force.

hope this !

im sorry if i went too in depth, i am going to include a link to a video on khan academy that will explain this.   hope that you like the video and that it explains more in detail to you.

that is the link.