It is raining hard on the A28 near Zwolle, visibility roughly 40 metres. May you switch on the rear fog light?

Yes — visibility is below 50 metres, so the condition is met

No — rain does not fall under the 50 m rule, not even at low visibility

Explanation

Wrong — trap: rain-and-fog confused. **<50 m due to fog/snow: on. Rain: never** (Book p.191).