题目内容：
根据下面资料，回答题 U．S．health officials are increasing surveillance measures at doctors' offices and internationalborders to guard against the spread of swine flu．Washington also has begun dispersing medicinefrom a federal stockpile．
The Centers for Disease Control and Prevention says there have been only mild cases of swineflu in the United States, but experts remain on guard．
Acting agency director, Richard Bess er, says the epidemic in Mexico prompted U．S．Doctorsto begin monitoring actively for possible infections．
"We are asking doctors when they see someone who has flu-like illness who has traveled to anaffected 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, Bess ersaid the extra detection efforts have enabled officials to find more infections than under normal circumstances．He also says he expects the numberof infections will rise and the illness will spread to other U．S．regions, as doctors continue to monitor the problem．
The Centers for Disease Control and Prevention says it does not recommend people travel toMexico, where the outbreak of swine flu is centered and more than 100 deaths have been reported．
But officials have not ordereda travel ban to the country．
Homeland Security Secretary Janet Napolitano says, instead, airlines have the option ofscreening passengers on flights from Mexico．
"We are letting air carriers and our employees at the gates on those flights make sure that they areasking people if they are sick; and if they are sick, that they should not board the plane, " she said．
Denise Komiewicz, 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 nations 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, " shesaid．"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 Iresbeen 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 responemeasures 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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