1. 다음 글에서 밑줄친 they와 it이 가리키는 것은?

 Protein are much more complex substances than carbohydrates or fats. They are built out of 22 different amino acids, ten of which are 'essential' as they cannot be made in the body. If you eat a steak, you are taking on a complete form of protein that has been made by the cow. Your body cannot use this and so has to break it back down into its component  parts before making it into a protein it can use. This is why meat can be hard to digest. 

① protein, cow

② proteins, steak

③ ten amino acids, your body

④ ten amino acids, steak

⑤ carbohydrates or fats, your body