With every word a child reads they decide if they know it.
Part C is about the value of comprehension.
On the track near "What's the meaning?" the orange cart carries the puzzle pieces 'b-o-l.' Its reader knows the three sounds for “ball.” But since the letters and sounds do not match, this doesn’t help with knowing the word. This child may easily get confused and combine sounds from other words, like in box #1 “My red bed” and box #2 “My red doll.”
On the other hand, the green cart shows how the child hears the sounds and knows how the vowels and consonants work together. This reader, speeding down her mental track, identifies the word quickly and comprehension grows!