You travel in a circle, whose circumference is 8kilometers, at an average speed of kilometer/hour. if you stop at the same point you started from, what is your average velocity

You skipped over a number in the question, and you didn't tell me what my average speed is. Lucky for you, my average speed has NO EFFECT on the answer to the question.

When you calculate velocity, you only use the straight-line distance between the start-point and the end-point. It doesn't matter what route the thing took to get there, or how much ground it actually covered.

If I travel in a circle and stop at the same point I started from, then the size of the circle doesn't matter, and neither does my speed. The distance between my start-point and my end-point is zero, and my average velocity is zero.

increases it

d. air pressure decreases as altitude increases.