在一定地域内，____与____所形成的统一的整体叫做生态系统．

某同学从象达去甘寨，前一半路程的平均速度是2m/s，后一半路程的平均速度是3m/s．则整个过程中的平均速度是____m/s．

己知 ，且满足 ，则 等于(     )

若圆关于直线x – y – 1 = 0对称的圆的方程是，则a的值等于（   ）

阅读理解
    Spring is an inviting season for outdoor activities. But be careful—the warm weather is also a hotbed for viruses(病毒) and disease.  
This March, the influenza(流行性感冒) A(H7N9) virus hit Shanghai, Anhui, Jiangsu and Zhejiang. It was reported that by April 11 at least 35 people had been infected (被感染的) with the H7N9 virus, and nine of them had died.
    The influenza A (H7N9) virus is one type of influenza A H7 viruses. Influenza A H7 viruses normally spread among birds.
This is the first time human infections with H7N9 viruses have been reported in China. Earlier reports were about H5N1 and H1N1.
All the three viruses are influenza A viruses but they are not the same. H7N9 and H5N1 are animal influenza viruses that could infect people. However, H1N1 viruses can be divided into two groups. One group can normally infect people and the other normally infects animals. Also, H1N1 can spread from human to human, but H7N9 probably can't, according to the World Health Organization (WHO).
Most people infected with H7N9 looked like they had the common flu. Some people had bad pneumonia(肺炎). They suffered from a fever, a cough and shortness of breath.
    But don't be afraid – it is not easy to be infected with the virus. One of the possible reasons is close contact with sick poultry (家禽) waste. People may also be infected through breath, according to National Health and Family Planning Commission.
Here are some tips from the WHO that can protect you from being infected.
    Wash your hands with soap and running water before you eat, after you use the toilet, and after touching animals or animal waste.
When coughing or sneezing cover your mouth and nose with a mask(口罩)，tissue(纸巾) or a sleeve.
    Get a good rest and do exercises, which will help make your body strong enough to stand up to the virus.