【正确答案】
B
【答案解析】[听力原文] International health officials don't know how far the dangerous Zika virus was spread. But with spring bringing warmer temperatures to the northern hemisphere, they're concerned that the mosquito that carry. Zika could bring the virus north of Brazil, the country hit hardest so far. Zika is not threatening to many adults who get it. Most don't even have symptoms. Those who do may have a few days of a fever, headache or muscle pain. But the unborn babies of pregnant women who get Zika are at risk of microcephaly, a disease that slows brain development and can cause lifelong problems. And officials are now linking Zika to premature birth, as well as eye problems and nervous system diseases in babies. There is no treatment and no cure for Zika. But researchers are racing to develop a vaccine for it, and they could be ready to test one later this year. Question: What is the international health officials' concern?