Here's a problem to solve: When a chicken egg was placed in a beaker of vinegar, bubbles formed on the eggshell. After twenty-four hours, the eggshell is no longer present. Which statement is an observation?

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

- A: Gravity made the egg sink to the bottom of the beaker.
 
- B: The vinegar caused the eggshell to dissolve.
 
- C: The egg was denser than the vinegar.
 
- D: Bubbles appeared on the eggshell.
 