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tokens: for such an effect. We point out that these X-ray reprocessing models cannot simultaneously explain several other aspects of AGN UV/optical variability, including the timescale dependent color variations (i.e., the lamp-post SSD can explain the variable spectra, but cannot account for the fact that the color variations are timescale dependent; see, e.g., Zhu et al. ( 2018 ) and the anti-correlations between variability amplitude and L (e.g., MacLeod et al. 2010 ; Sun et al. 2018b ). Below, we discuss the alternative disk-corona magnetic coupling scenario (Sun et al. 2020a ), which has shown to be successful in reproducing the color variations and the dependences of the variability amplitude upon L (Sun et al. 2020b ), to understand our results in Figure 1 . 3.2. Implications for the Magnetohydrodynamic Accretion Physics Theoretically speaking, disk temperatures ( T ) should vary in response to the time-dependent magnetohydrodynamic (MHD) turbulent heating with a response on the thermal timescale ( τ TH ). For an SSD, , where α , G , and R are the dimensionless viscosity parameter, the gravitational constant, and the distance to the central black hole, respectively. Therefore, the short-wavelength emission, which is produced by the inner hotter plasma, has a smaller response thermal timescale than the long-wavelength emission. The differences in the response thermal timescale would add an additional inter-band time lag ( τ add ), beyond the light-travel time lags. On short timescales ( t dur ), the observed UV/optical variations are mainly produced in the similar small- R regions with τ TH ≤ t dur as the gas in these regions can vary its temperature significantly, and τ add is small and negligible (and vice versa). For an SSD, at the characteristic radius R λ (with k B T ( R λ ) = hc / λ , where k B and h are the Boltzmann and Planck constants, respectively; and ), τ TH ( R λ ) scales as α −1 L 0.5 λ 2 . Therefore, τ add would only be prominent for AGNs with small L (i.e., short τ TH ( R λ )) or large t dur . Below, we consider a specific thermal-fluctuation model to qualitatively reproduce the observed result. In earlier work, we proposed the corona-heated accretion-disk reprocessing model (hereafter the CHAR model) by assuming the accretion disk and the extremely compact X-ray corona are magnetically coupled (Sun et al. 2020a ). Magnetic fluctuations in the corona can alter the disk surface magnetic field, drive variations in the disk MHD turbulent dissipation, and eventually induce coherent multi-band variations. We have predicted that τ dif increases with decreasing L for sources with similar monitoring durations (see Figures 8 and 16 of Sun et al. 2020a ). The first two model parameters, M BH and , are fixed according to the observational constraints. The only free parameter, α , might be 0.1–0.4 according to studies of the outbursts of dwarf nova or soft X-ray transients (e.g., King et al. 2007 ). We fix 10 10 Note that the CHAR model