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. It would be impossible to run epidemiological studies to look for a relationship between cell phone use and brain cancer development, because the wide array of confounding variables would make any possible outcome so unreliable as to be practically inconclusive, so epidemiologists can only point to declining brain-cancer rates in general in rich countries that had widespread cellphone use, along with laboratory studies on mice which have never shown tumor growth as a result of cell phone emissions, and say that the effect, if it exists, cannot be a large one.
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.