What cancels the permit for use of a motorized bicycle?

The combination selected is correct because both actions—clearing out the silencer and modifying the gear ratio—alter the performance characteristics of the motorized bicycle beyond legal limits as prescribed in German traffic laws.

Specifically, clearing out the silencer can increase noise levels, which may exceed permitted sound limits, while altering the gear ratio can affect the speed and handling of the vehicle, making it more akin to a motorcycle rather than a bicycle. Both of these modifications can impact safety and compliance with regulations, leading to the cancellation of the permit for use.

Fitting a second rear-view mirror, however, is a modification that does not invalidate the permit and may actually enhance safety by improving visibility. Understanding these distinctions is key to obeying traffic regulations and ensuring the vehicle remains roadworthy.