Practice: A 6-year-old boy gets very angry at his parents because he is only allowed to have four Reese’s Peanut Butter Cups after dinner (he wants six—one for each year of his life). In protest, he threatens to hold his breath until he passes out. He starts to hold his breath, causing CO2 to accumulate in his bloodstream. Which of the following forms of CO2 will accumulate to the highest levels in his blood?