Read this: British night air defences were in a poor state. Few anti-aircraft guns had fire-control systems, and the underpowered searchlights were usually ineffective against aircraft at altitudes above 12,000 ft (3,700 m). In July 1940, only 1,200 heavy and 549 light guns were deployed in the whole of Britain. Of the "heavies", some 200 were of the obsolescent 3 in (76 mm) type; the remainder were the effective 4.5 in (110 mm) and 3.7 in (94 mm) guns, with a theoretical "ceiling"' of over 30,000 ft (9,100 m) but a practical limit of 25,000 ft (7,600 m) because the predictor in use could not accept greater heights. The light guns, about half of which were of the excellent Bofors 40 mm, dealt with aircraft only up to 6,000 ft (1,800 m). Although the use of the guns improved civilian morale, with the knowledge the German bomber crews were facing the barrage, it is now believed that the anti-aircraft guns achieved little and in fact the falling shell fragments caused more British casualties on the ground.

British night air defenses were effective to what height?
What is the answer? (If it cannot be answered, return "unanswerable")
12,000 ft (3,700 m)