How to keep cut apples fresh?

We are learning about changes in food. I investigated how to keep cut apples fresh by placing them in different solutions.
What I needed:
4 clear plastic cups
1 teaspoon salt
1 teaspoon sugar
1 apple cut into 4 pieces
What I did:
1. Cup 1 - Fill with 3/4 water and place a piece of apple.
2. Cup 2 - Fill with 3/4 water, a piece of apple and 1 teaspoon of salt.
3. Cup 3 - Fill with 3/4 water, a piece of apple and 1 teaspoon of sugar.
4. Cup 4 - Place a piece of apple.
What I observed after 3 hours:
The apple in cup 1 was brown.
The apple in cup 2 was only a light brown compared to all the other apples.
The apple in cup 3 with the sugar solution was browner than the apple in cup 2 .
The apple in cup 4 with no solution was the brownest from all the other cut apples. This is called oxidation and is caused by the enzymes in the apple and oxygen.
The best solution to keep apples fresh is salt.
By Timote