Which statement correctly explains why $(x+3)$ and $(x-3)$ cannot be canceled termwise in a rational expression?

• A. They represent the same polynomial evaluated at different points, so cancelation depends on substituting a test value to check equality before canceling
• B. They can be canceled termwise because the variable terms match and constants differ only by sign, so cancelation is allowed when numerators and denominators are linear
• C. They are opposites and always simplify to -1, so cancellation is unnecessary but valid in all algebraic manipulations
• D. They are not identical factors, so term‑by‑term cancellation is not valid in a rational expression

Answer: (D)