Answer :
I am not sure but I think it would be chemicals because you can see evrything except chemicals
I think that it would be machine guns because when a soldier was moving the machine guns would be in the trenches and they would have a little protection around them making them blend in more. Thus, when the troops would get close enough the machine guns would open fire and possibly kill the whole platoon. But I could also see where you could get chemical weapons from this but they were launched with artillery and would often explode on impact releasing the chemicals which you could see though. So the most correct answer would be machine guns. Hope this helped you out.