Which of the following quantities are unknown? initial separation of the particles final separation of the particles initial speed of the proton initial speed of the alpha particle final speed of the proton final speed of the alpha particle mass of the proton mass of the alpha particle charge of the proton charge of the alpha particle Enter the letters of all the correct answers in alphabetical order. Do not use commas. For instance, if A, C, and D are unknowns, enter ACD.