设{an}递增等差数列，前三项的和为12，前三项的积为48，则它的首项是（） A

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Tuberculosis(肺结核，TB) killed 1,300,000 people arould the world in 2007. In addition, almost half a million people who were infected with tuberculosis and with HIV also died. Those were listed as  HIV deaths.
About one-third of all people are infected with tuberculosis. But the body’s natural defenses are usually strong enough to prevent an active case. Even so, the bacteria remain in the body. If the immune system weakens at any point, they begin to spread and then attack.
The bacteria that cause TB usually settle in the lungs. They spread through the air when the person coughs or even sings and talks.
One of the most important things is to identify cases quickly. The patients need to be kept away from other people and begin treatment as soon as possible.
Multidrug resistant tuberculosis, orMDR-TB, will not get better with antibiotics normally used for tuberculosis. So doctors must use stronger, “second line “ drugs when the first ones fall.
Now, researchers say they have found a much faster way to identify drug-resistant TB. The study’s lead author is Graham Hatfull at the University of Pittsburgh in Pennsylvania.
The scientists used viruses called bacteriophages to speed the process. These viruses attack bacteria. The researchers injected them with a gene that produces green light. They also injected some with first line antibiotics and others with second line drugs.
Then they combined the bacteriophages with TB bacteria. If the bacteria produces green light, it means they are drug-resistant. The researchers say a clinic worker could identify the green light with equipment easy to get in many clinics. Test results would not have to wait for the bacteria to grow in a lab far away. For now , the test itself needs more testing. But Professor Hatfull is hopeful this will take months and not years.
Title:__1___

The __2__of people infected withTB	One-third of people around the world
When to begin to spread and attack	When the immune system __3___
Where bacteria __4__TB uaually settle	In the lungs
How to __5__	Through the air
_____6__	___7__cases quickly. Keeping the 8_ away from people Beginning __9__as soon as possible Using _10_, second-line drugs to treat MDR-TB