Final answer:
Gas detectors and flame detectors are best for warehouses, fuel storage areas, paint shops, and outdoor or semi-enclosed areas.
Step-by-step explanation:
For warehouses, fuel storage areas, paint shops, and outdoor or semi-enclosed areas, the best detector would be a combination of gas detectors and flame detectors. Gas detectors are used to detect the presence of flammable or toxic gases, such as natural gas or ammonia, while flame detectors can quickly detect the presence of an open fire or flame.