阅读下列短文，从每题所给的四个选项(A、B、C和D)中选出最佳选项。
Have you ever wondered how many ants (蚂蚁) there are on Earth? Scientists at the University of Hong Kong certainly have. In a new study, they estimated (估计) the number of the world's ants is about 20,000,000,000,000,000 (20 with 15 zeroes)!
That means for every human, there are nearly 2.5 million ants. If you put all the ants on a scale (秤), they'd weigh more than all the wild birds and mammals (哺乳动物) on Earth.
How did they count such a big number? In the past, many scientists have counted ants in different areas. They collected fallen leaves and earth and dried them to force the ants to come out for counting. Based on 465 studies that cover all continents, the new study made the estimation.
According to Wang Runxi, one of the authors, the number and distribution (分布) of ants can be valuable predictors of environmental changes.
"Ants play an important role in the ecosystem (生态系统)," Wang told Science and Technology Daily. "They help loosen the soil (松土), spread seeds, break down things and form a part of the food chain (食物链). They are everywhere on Earth except for the coldest places."
"Compared with big animals, little ants are more sensitive (敏感的) to environmental changes," Wang added. Most ants live in tropical (热带) and subtropical (亚热带) areas. But in recent years, ants have been moving toward the two poles because of global warming. Changes in forests, such as too much logging, also affect the number of ants, the study showed.
"Our global map of ants allows us to better understand and predict ants' responses to worrying environmental changes," scientists wrote in the study.