题目内容：
根据以下资料，回答题：U．S．health officials are increasing surveillance—measures at doctors’ offices and international borders to guard against the spread of．swine flu．Washington also has begun dispersing medicine from a federal stockpile．
The Centers for Disease Control and Prevention says there have been only mild cases of swine flu in the United States，but experts remain on guard．
Acting agency director，Richard Besser,says the epidemic in Mexico prompted U.S.doctors to begin．monitoring actively for possible infections．
“We are asking doctors when they see someone who has flu—like illness who has travel to an affected region，to do a culture，take a swab in the Nose and send it to the lab so we can see:is it influenza．is it this type?”he said．
Speaking Sunday at the White House，Besser said the extra detection efforts have enabled offi-cials to find more infections than under normal circumstances．He also says he expects the number of infections will rise and the illness will spread to other U．S．regions，as doctors continue to mo-nitor the problem．
The Centers for Disease Control and Prevention says’it does not recommend people travel to Mexico,where the outbreak of swine flu is centered and more than 100 deaths have been reported．But officials have not ordered a travel ban to the country．
Homeland Security Secretary Janet Napolitano says，instead，airlines have the option of screening passengers on flights from Mexico.
“We are letting air carriers and our employees at the gates oil those flights make sure that they：are asking people if they are sick；_and if they are sick，that they should not board the plane．”she said.
Denise Korniewicz，an infectious disease expert at：the University of Miami，says officials should take bolder steps to screen passengers at international borders，as Japan and other Asian na-tions are doing．
“We have a very transient population here．And Japan has taken a lot of precautions．What Japan is doing is they are making everyone take a temperature when they get off the airplane."she said．“As far as I am concerned．I think that is a good idea．”
U．S．officials say they are holding off on more aggressive actions because the outbreak has been limited in the United States and they do not want to cause a health scare．
Korniewicz says around the country health centers are putting in place emergency response measures aimed at limiting disease outbreaks．
What promotes American doctors to begin monitoring actively for possible infections? A．There have been serious cases of swine flu in the United States．
B．U．S．health officials are increasing surveillance measures at doctors’offices．
C．The epidemic happened in Mexico.
D．Experts remained on guard．
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