Question: In the context of EVS (Enhanced Voice Services) codec's Forward Error Correction (FEC), which frame class can ONLY follow a frame classified as UNVOICED CLASS or UNVOICED TRANSITION CLASS, indicating a potential voiced onset that is not yet fully established for voiced concealment?
Option 1: VOICED TRANSITION CLASS
Option 2: UNVOICED TRANSITION CLASS
Option 3: ONSET CLASS
Option 4: VOICED CLASS
Explanation: The correct answer is ONSET CLASS because, according to the EVS codec's FEC strategy, an ONSET CLASS frame represents a voiced onset following an unvoiced or unvoiced transition segment, where the onset is sufficiently developed for voiced frame concealment techniques but differs in recovery strategy from stable VOICED CLASS frames. This specific sequencing rule (only following UNVOICED CLASS or UNVOICED TRANSITION CLASS) uniquely identifies ONSET CLASS among the given options.