26. Which of the following statements about isotopes are correct? (A) Different isotopes of the same element contain different numbers of electrons. (B) The mass number of an isotope equals the total number of protons and neutrons in its nucleus. (C) The atomic masses of carbon-12 and carbon-13 are exactly 12 u and 13 u, respectively. (D) The atomic mass listed for an element in the periodic table is a weighted average of the masses of its naturally occurring isotopes. (E) Isotopes are represented by a subscript mass number to the left of the element symbol, for example, ²³⁵U.