Why must you reduce your speed now?

Your answer is correct because the main reason to reduce speed in this situation is the truck ahead. According to German traffic rules, when you see slower vehicles or traffic congestion ahead, you must adjust your speed accordingly to maintain a safe following distance and avoid collisions. The speed limit sign alone does not always require you to slow down unless it indicates a lower limit than your current speed, but here the crucial factor is reacting to the vehicle in front. The presence of the blue car behind you does not require you to slow down; instead, your focus must be on the traffic ahead to ensure safety. Always observe and adapt your speed primarily based on the situation directly in front of your vehicle.