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Consider an $FM$ signal $f(t)= \cos\left [ 2\pi f_{c} t+\beta _{1}\sin2\pi f_{1}t+\beta _{2}\sin2\pi f_{2}t\right ].$ The maximum deviation of the instantaneous frequency from the carrier frequency $f_{c}$ is

- $\beta _{1}f_{1}+\beta _{2}f_{2}$
- $\beta _{1}f_{2}+\beta _{2}f_{1}$
- $\beta _{1}+\beta _{2}$
- $f _{1}+f _{2}$