Question: When switching from a primary mode to an AMR-WB IO (Codec for Enhanced Voice Services) encoded frame that uses CELP (Code-Excited Linear Prediction), what must be done to the memories of the CELP AMR-WB IO encoded frame if it is preceded by a primary mode encoded frame at a 16 kHz internal sampling rate?
Option 1: Only reset the gain quantization memory to -14.0 and perform processing as described for switching from a CELP primary frame.
Option 2: Convert previous quantized end-frame LSFs to ISFs, convert previous quantized end-frame LSPs to ISPs, and then apply specific processing for MDCT primary frames.
Option 3: Limit the index of last encoded CNG energy to 63, reset AR model LP quantizer memory, but do not perform any sampling rate-specific processing.
Option 4: Force a SID frame, update all memories as if switching from an MDCT primary frame, and ignore the internal sampling rate.
Explanation: According to the provided context, when an AMR-WB IO frame is preceded by a CELP primary frame at a 16 kHz internal sampling rate, the specific processing described for this scenario (subclause 5.4.4) should be performed, in addition to the general memory updates required for switching to CELP AMR-WB IO. This indicates that the correct approach involves handling the switch with considerations for the CELP primary frame at the specified sampling rate, without unnecessarily applying MDCT-specific processing or ignoring critical memory resets.