(a)
In order to find the products that sold more in 2020 than in 2021, we need to find which ones in the graph have the purple bar bigger than the red/white bar.
Looking at the graph, the products are Video games and DVDs.
(b)
Now, let's analyze the purple bar (bar that is related to the year of 2020) of each product, and check which ones are below the mark of 800 products.
Looking at the graph, the products are Video games, Radios and DVDs.
(c)
Let's add the values of the purple bar and red/white bar for the radios.
The purple bar is approximately 400 and the other bar is approximately 500, so the total number of radios sold is approximately 900 radios.