21. In instrumental analysis, the internal standard method is often preferred over external calibration when sample loss or injection variability is expected. Which of the following statements regarding the properties and selection of an internal standard is correct?
(A) The internal standard should be a substance that is already present in the original sample matrix to ensure compatibility.
(B) The primary purpose of an internal standard is to compensate for matrix effects that suppress or enhance the analyte's signal.
(C) An ideal internal standard should have a chemical structure and physical properties similar to the analyte, but its signal must be well-resolved from the analyte's signal.
(D) Adding a higher concentration of the internal standard will directly increase the sensitivity (slope) of the calibration curve for the analyte.
(E) If the instrument's sensitivity drops by 20% during a run, the internal standard method will fail because the response factor is no longer constant.