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The maximum speed allowed outside built-up areas for motor vehicles that tow trailers or semitrailers is 10 km/h lower than the maximum speed allowed for the category to which the towing motor vehicle belongs.

Therefore, when you are driving a car from category B that tows a semitrailer on a European national road (E) outside built-up areas, the maximum speed at which you can drive is 100 km/h -10 km/h = 90 km/h.

The correct answer is: C

Recommendations:

Obligations and prohibitions when towing vehicles - Audio-Video Lesson --> Road Traffic Code - Obligations and prohibitions in the event of technical failure or when towing vehicles

The maximum speeds allowed on different categories of roads depending on the motor vehicle driven - Audio-Video Lesson --> Road Traffic Code - Driving speed

OUG* - Article 50

(1) The maximum speeds allowed outside built-up areas for the categories of motor vehicles provided in annex no. 1 are the following:
a) 130 km/h on motorways, 120 km/h on expressways, 100 km/h on European national roads (E) and 90 km/h on the other categories of roads, for motor vehicles in categories A and B;
[...]
(2) The maximum speed allowed outside built-up areas for motor vehicles that tow trailers or semitrailers is 10 km/h lower than the maximum speed allowed for the category to which the towing motor vehicle belongs.
[...]

* OUG = GOVERNMENT EMERGENCY ORDINANCE no. 195 of December 12, 2002 updated (Road Traffic Code)