\textbf{189.} The equation of the magnetic flux passing through a coil with 60 loops is, in SI units: $\phi = 4\times10^{-3}\cos100\pi t$
The average magnitude of the induced EMF in the coil over the time interval $t_1 = \dfrac{1}{400}\,\text{s}$ to $t_2 = \dfrac{1}{100}\,\text{s}$ is how many volts?
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(1) $2.4$ \quad (2) $4.8$ \quad (3) $24$ \quad (4) $48$
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