The word "their" in paragraph 3 refers to ___________

Câu hỏi: Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 30 to 34.
In One Cubic Foot In any environment — forest, mountain, or water — you always see the big animals first: birds, mammals, fish. But under your feet, on land or in the water, there are many smaller organisms: insects, tiny plants, miniature sea creatures. They seem unimportant, but, in fact, these sea creatures and ground dwellers are "the heart of life on the Earth," says naturalist E.O. Wilson. Without them, our world would change dramatically.
Most organisms on the Earth live on the ground or just below it. Here, they are part of an important cycle. Plants and animals fall to the ground when they die. Later, tiny insects and other organisms break down the dead plant and animal material. This process eventually returns nutrients to the soil and gives plants energy. Plants can then help to maintain a healthy environment for humans and other animals.
Despite their importance, scientists know very little about most ground organisms. To learn more, photographer David Liittschwager went to different places around the world, including a forest, a river, a mountain, and a coral reef. In each place, he put a green 12-inch cube on the ground or in the water. Then he and his team counted and photographed the organisms that lived in or moved through the cube. Often they discovered hundreds, some only a millimeter in size. "It was like finding little gems," he says. In the coral reef in French Polynesia, he saw thousands of creatures in the cube and photographed 600. The team identified as many as possible, but it was difficult. Many of the animals they found were new species.
The word "their" in paragraph 3 refers to ___________
A. scientists
B. ground organisms
C. cubes
D. nutrients