Which of the Following Is NOT a Reason Cited for Inconclusive

Which of the following is NOT a reason cited for inconclusive findings on cell phones and brain tumors

A) Cell phone technology has changed over the past 30 years.
B) Newer cell phones use analog technology, which permits higher-frequency radio signals, posing fewer health risks.
C) First generation cell phones emitted low-frequency radio waves that some researchers believe are harmful after prolonged exposure.
D) Research is not yet available on the effects of long-term use of digital cell phones.