题目内容：
完成选项A chemical important of brain development may play a role in explaining why some people are genetically (由基因决定地)likely to suffer from anxiety and could lead to new treatments, the U．S． researchers said．
They said highly anxious rats which were kept had very low levels of a brain chemical calked fibroblast growth factor 2 or FGF2, compared with rats that were more relaxed． But when they improved the anxious rats' living conditions -- giving them new toys to explore and a bigger cage to live in -- levels of this brain chemical increased and they became less anxious．
"The levels of this brain chemical increased in response to the experiences that the rats were exposed to． It also decreased their anxiety"． Javier Perez of the University of Michigan said in a telephone interview． "It made them behave the same way as the rats that were relaxed", he said．
In a former study of people who were severely depressed before they died, the team found the gene that makes FGF2 was producing very low levels of the growth factor, which is known primarily for organizing the brain during development and repairing it after injury．
Perez thinks the brain chemical may be a marker for genetic vulnerability (脆弱性)to anxiety and depression． But it can also respond to changes in the environment in a positive way, possibly by preserving new brain cells．
While both the calm and anxious rats produced the same number of new brain cells, these cells were less likely to survive in the high-anxiety rats, the team found． Giving the rats better living conditions or injecting them with FGF2 helped improve cell survival．
"This discovery may pave the way for new, more specific treatments for anxiety that will not be based on sedation(药物镇静), but will instead fight the real cause of the disease," Dr． Pier Vineenzo Piazza from France said in a statement．
We know from the passage that with the levels of FGF2 decreasing, the rats will A．die of anxiety soon
B．suffer from a headache
C．become more relaxed
D．become more anxious
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