Here's a problem to solve: A researcher uses several procedures to separate a rock sample into different chemicals. A mass of 50 grams of one chemical is produced. If this chemical cannot be separated into other chemicals, then it is best described as

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

- A: a compound.
 
- B: a mixture.
 
- C: a solution.
 
- D: an element.
 