With no explanation, label text_A→text_B with either "DON'T KNOW", "NO" or "YES".
text_A: The World Health Organization has classified radio frequency electromagnetic radiation as Group 2B - possibly carcinogenic. This group contains possible carcinogens such as lead, DDT, and styrene. Epidemiologists have run studies to try to find any relationship between the development of tumors in the brain and use of cell phone during participants' lives, and these studies have not drawn firm conclusions, and have only been able to say that is any such effect exists, it can't be significantly large.
text_B: Does the passage still leave open the possibility that cell phones could be dangerous, but that the effect has been hidden by medical advances that have reduced brain-cancer from other causes?
YES.