单选题：Passage 1Questions are based on the following passage.Spend

Passage 1
Questions are based on the following passage.
Spending 50 minutes with a cell phone close to your ear is enough to change brain cell activity in the part of the brain closest to the antenna (天线).But whether that causes any harm is not clear, scientists at the National Institute of Health said at a conference last month, adding that the study will not likely settle concerns of a link between cell phones and brain cancer."What we showed is glucose(葡萄糖) metabolism(代谢) (a sign of brain activity) increases in the brain in people who were exposed to a cell phone in the area closest to the antenna," said Dr.Nora Volkow of the NIH, whose study was published in the Journal of the American Medical Association.(76) The study was meant to examine how the brain reacts to electromagnetic fields caused
by wireless phone signals.
Volkow said she was surprised that the weak electromagnetic radiation(电磁辐射) from cell
phones could affect brain activity, but she said the findings do not shed any light on whether cell
phones cause cancer."This study does not in any way indicate that.What the study does is to show the human brain is sensitive to electromagnetic radiation from cell phone exposures." Use of the devices has increased dramatically since they were introduced in the early 1980s, with about 5 billion cell phones now in use worldwide.
Some studies have linked cell phone exposure to an increased risk of brain cancers, but a large study by the World Health Organization did not offer a clear answer to this.Volkow' s team studied 47 people who had their brain examined while a cell phone was turned on for 50 minutes and another while the phone was turned off.While there was no complete change in brain metabolism,
they found a 7 percent increase in brain metabolism in the region closest to the cell phone antenna
when the phone was on.
(77) Experts said the results were interesting, but urged that they be understood with great care." Although the biological significance, if any, of increased glucose metabolism from too much cell phone exposure is unknown, the results require further investigation," Henry Lai of the
University of Washington in the U.S.and Dr.Lennart Hardell of University Hospital in Sweden,wrote in an article in JAMA．"Much has to be done to further investigate and understand these effects." They wrote.
According to the passage, which of the following is TRUE? A．Cell phone use is dangerous.
B．.Cell phone use causes cancer.
C．The human brain is an electromagnetic field.
D．There are about 5 billion cell phone users in the world right now.