You are unique. There are 6.5 billion people in the world but no other person is exactly like you. Besides people, there are millions of other living things. Every living thing is different from each other. Every living thing is a unique combination of characteristics.Why is each living thing unique? Where do its characteristics come from? Do people receive characteristics from their mothers and fathers? How? The work of Gregor Mendel was especially important to help us to answer these questions.Mendel studied plants, especially pea plants. In some way they were all the same. For example, they all had flowers. But some had red flowers, and others had white flowers. Some were tall, others were short. Mendel was especially interested in the differences. He wondered why each plant was of certain color, shape, size. He experimented with thousands of pea plants, with a careful record. And he discovered some interesting things. He mated different pea plants. First, he mated a red flower plant with a white flower plant. All of the offspring plants had red flowers. Mendel was curious about what happened to the white. Then he mated two of the offspring plants. Every time he did this, they produced three red plants and one white plant. There was the white again. So Mendel knew that even red plants somehow passed on whiteness. Mendel decided that pea plants carried factors, and the parents passed these factors onto their offspring.Today we call these factors genes. Genes are tiny pieces of matter. They carry information from parents to offspring. Now we know that every person is unique and one reason for this is that every person is a unique combination of genes.In the first paragraph, the author mainly tells us that____________.
选项：

A: no other person is exactly like you
B: the total population of the world is 6.5 billion
C: every living thing is unique
D: different people have different characters