题目

Scientists investigated why Ebola virus is so deadly when it spreads from animals to humans and then from human-to-human contact. The research team looked at the Zaire Ebola virus in an animal system to understand how it gains strength. This virus is responsible for the current outbreak in West Africa. They found that initially the animal systems were not affected by the virus, but succeeding transmission(传送) into other animals caused the virus to “hot up” and become more severe.The team analyzed the viruses at different stages and were able to identify several changes in its genetic m Carl hopes to win the tennis game one day. That is               he has been practicing hard.A．whyB．howC．becauseD．where