[QA] According to the passage, the numerical value of the concentration level of a substance is only useful if ________.

Question:

According to the passage, the numerical value of the concentration level of a substance is only useful if ________.

• A.
  the other substances in the area are known
• B.
  it is in a localized area
• C.
  the natural level is also known
• D.
  it can be calculated quickly