Anti-lock brakes (abs)
a. prevent skidding and allows drivers to steer during an emergency braking situation.
b. sense a skid and automatically apply the brakes.
c. stop the vehicle faster than normal brakes.
d. all of the above

A. prevent skidding and allows drivers to steer during an emergency braking situation.

Explanation:

In an urgent stopping case, anti-lock brakes avoid skidding and allow drivers to steer. ABS can also help improve automobile balance (evitating spinouts), steering (that is directing the vehicle where the driver needs it to go) and braking (distance needed to stop the vehicle). Hence, the correct answer is A.

d.7.5cm is your answer

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.