Used in machine guns since World War I, tracer ammunition contains modified bullets packed with a pyrotechnic charge that ignites upon firing, illuminating the bullets. This technology enables a shooter to check a bullet’s trajectory and adjust their aim accordingly. Since the tracer path can also betray a shooter’s position, many modern tracers are designed to become visible only once they have traveled a set distance. Why were tracers especially useful against Zeppelins in World War I? Discuss