Can u guys plz answer the question 20,21,22

First, it is helpful to identify what each image represents.

Number 1. is an image of an Artery. I know this because Arteries have a thick outer wall (the dark outer circle), a small lumen ( innermost circle the empty space in a tubular structure), and in between those inner and outer layers arteries have a thick layer of muscle and elastic fibers. 

Number 2. This image is of a Vein. I know this because one of the characteristics of veins is having a thin outer wall and a thin layer of muscle between the thin outer wall and the large lumen. (allowing it form that funny shape).

Number 3. This pictures a Capillary. I know this because capillaries are the smallest blood vessels. They are made of a single layer of cells. 

Next, It is important to know the purpose related to each structure.
 Arteries- sometimes called "pressure reservoirs" because they maintain the high pressures from which the blood is pumped from the heart to the body. The high pressures that the arteries handle explains why they have suck thick and muscular walls. 
Viens- Responsible for carrying blood back to the heart. They do this using a system of valves so that the blood doesn't fall to the feet due to gravity.(because forward blood pressure is very low compared to the arteries.)
Capillaries- the smallest blood vessel their single cell layer allows for gas exchange. Put O2 into the body tissues and remove their CO2.

Knowing this information, these are the answers I got:
20. 3 - capillaries are responsible for gas exchange.
21. 2.- veins use a system of valves to transport blood back to the heart
22. 1. - Arteries have thick walls to withstand the high pressures that they maintain.

Make sense? Any further questions?