Word Choice

Here is a cloze question designed by a Chinese teacher. Can you tell me why the answer is the former rather than the latter? Upon what do we base our choice?

Some thinkers before Darwin believed that species of animals or plants could change over time and become new species. That process is now called evolution. But no one had clearly seen all the evidence for evolution in nature. And no one had figured out how it could happen, or explain it so 1____(thoroughly, precisely)that others would understand it. Darwin was the man who did all that.

He was the best biologist ever - one of the world's top experts on plants and pigeons, earthworms and orchids, beetles and barnacles. And yet he wasn't 2. ___(trained, expected)to be a scientist. He was never a professor at a university. He was just a careful observer with a curious mind, a sharp thinker and clear writer.

But it was Darwin who fully explained evolution of new species by this method, which he called "natural selection." In 1859, he spelled out the details in a(n) 3____( book, article)titled On the Origin of Species, which electrified the scientific world - and the public as well. Many people didn't like it. But it became the most important book in the history of biology.

Today almost all biologists agree with his basic ideas. But they are still investigating evolution's details. Now scientists believe that although evolution often happens slowly and gradually, sometimes it also can happen 4.____( quickly, suddenly). Moreover, natural selection isn't the only way that life forms can change. And natural selection isn't open just for individual organisms, but also for whole species or even the genes in the body that are res