Here's a problem to solve: Sugars are necessary for human cell function. Which of the following are human cells not capable of doing?

Among the 4 following options, which is the correct answer?

- A: attaching sugars to protein molecules
 
- B: building larger molecules from sugars
 
- C: creating simple sugars from smaller molecules
 
- D: breaking down complex carbohydrates into simple sugars
 