Normal N2 Sleep — Spindles and K-Complexes

During a polysomnography teaching session, a 24-year-old healthy medical student is being recorded. The technician identifies a segment marked **B** on the tracing that displays short bursts of 12–14 Hz activity lasting 0.5–1 second, along with biphasic high-amplitude sharp-then-slow waves over the central-vertex electrodes, superimposed on a background of low-voltage mixed-frequency activity. Which of the following sleep stages is being recorded in segment **B**?