Question: In the EVS codec, for a frame to be classified as a tonal frame in Step c), which of the following conditions must **ALL** be true regarding its features?
Option 1: Time-domain stability feature > 0.072, Spectral centroid feature < 1.2, and at least one spectral flatness feature < its threshold.
Option 2: Time-domain stability feature < 0.072, Spectral centroid feature > 1.2, and at least one spectral flatness feature < its threshold.
Option 3: Time-domain stability feature > 0.072, Spectral centroid feature > 1.2, and all spectral flatness features > their thresholds.
Option 4: Time-domain stability feature < 0.072, Spectral centroid feature < 1.2, and at least one spectral flatness feature > its threshold.
Explanation: According to the detailed algorithmic description for the EVS codec, specifically in Step c) of the tonal frame determination process, a frame is considered tonal if three conditions are met: (1) the time-domain stability feature is **smaller than 0.072**, (2) the spectral centroid feature is **greater than 1.2**, and (3) at least one of the three spectral flatness features is **smaller than its threshold**. This ensures the frame exhibits characteristics consistent with tonal signals, such as stability, high-frequency content, and non-uniform spectral distribution.