Question

what is a necessary tool investigators need to detect trace metal evidence?
a fan
luminol
a plastic spray bottle
a flashlight

Explanation:

Luminol is a chemical that reacts with iron in blood (not metal) to emit light. A fan and a plastic spray bottle are not relevant for detecting trace metal evidence. A flashlight can be used to detect trace metal evidence as it can help in visual inspection under proper lighting conditions.

Answer:

A flashlight