After studying agriculture, medicine, and fisheries in college, John went back to observing nature and asking questions.

这是2023年新全国卷1阅读B中的一个句子。请问这里为什么不用 go back to do，而不用go back to doing 呢？

附全文：

When John Todd was a child, he loved to explore the woods around his house, observing how nature solved problems. A dirty stream, for example, often became clear after flowing through plants and along rocks where tiny creatures lived. When he got older, John started to wonder if this process could be used to clean up the messes people were making.

After studying agriculture, medicine, and fisheries in college, John went back to observing nature and asking questions. Why can certain plants trap harmful bacteria (细菌)? Which kinds of fish can eat cancer-causing chemicals? With the right combination of animals and plants, he figured, maybe he could clean up waste the way nature did. He decided to build what he would later call an eco-machine.

The task John set for himself was to remove harmful substances from some sludge (污泥). First, he constructed a series of clear fiberglass tanks connected to each other. Then he went around to local ponds and streams and brought back some plants and animals. He placed them in the tanks and waited. Little by little, these different kinds of life got used to one another and formed their own ecosystem. After a few weeks, John added the sludge.

He was amazed at the results. The plants and animals in the eco-machine took the sludge as food and began to eat it! Within weeks, it had all been digested, and all that was left was pure water.

Over the years, John has taken on many big jobs. He developed a greenhouse-like facility that treated sewage (污水) from 1,600 homes in South Burlington. He also designed an eco-machine to clean canal water in Fuzhou, a city in southeast China.

“Ecological design” is the name John gives to what he does. “Life on Earth is kind of a box of spare parts for the inventor,” he says. “You put organisms in new relationships and observe what’s happening. Then you let these new systems develop their own ways to self-repair. ”