A food handler is slicing roast beef continuously on a slicer for $6$ hours. After $4$ hours, the roast beef is removed and set aside while the slicer parts are washed, rinsed, and sanitized. The meat is then put back on the slicer to continue slicing. What is the most serious risk of this procedure?

a$)$ Time$-$temperature abuse of the roast beef

b$)$ Physical contamination from the cleaning process

c$)$ Chemical contamination of the slicer with the sanitizing solution

d$)$ Cross$-$contamination of the roast beef with the sanitizing solution