_____ is often the first indicator of fire, so a system of automatic detectors should be used. 3rd Class A2 Chapter 15 _____ is often the first indicator of fire, so a system of automatic detectors should be used. a. Heat. b. Smell. c. Carbon dioxide. d. Flames. e. Smoke. Answer: E Learn More : Share this Share on FacebookTweet on TwitterPlus on Google+