A person or substance that is responsible for creating a reaction or event would best be described as a (n)

Hi, my name is maya gupta.i'm here at a bank robbery that happened a few minutes ago.a man named tryce ramder is suspected to be the robber.i am here with daniel cottle to describe what happened."it was terrifying. we all just heard the door break and turned around. some man just had a gun and told us to freeze and drop our phones. luckily we had cameras. continuing on, the man walked up to a worker, and said "give me the money or i'll shoot you." i did not believe what i heard. after a few minutes, we heard sirens, and the robber looked fearful, yet so angry. so he took out a bomb and threw it near us. i was in the military, so i know what to do, so i told everyone to move away atleast and duck down and cover their ears. i almost got affected, but some woman was able to grab me and put me down, and cover my ears. she got affected, she passed out, and was taken to the hospital. i am gonna visit her now to her and give her gifts for what she did." you daniel cottle, let's all hope this woman will be alright. i am maya gupta and we are signing off. (sorry if it's too long, maybe this will you get an idea? )