下列现象中，体现生物与环境温度相适应的是（    ）

    Today is Tree- Planting Day. I go to the park with my parents. At about ten o'clock we start to plant trees. We needn't to dig holes because the people in the park dug them before. We just put the young trees into the holes. My father fills the holes with me. My mother carries water for the young trees. We are so tired when we finish the work. All of us hope the trees will grow well. I hope I can plant trees next year.

在$△ABC$中，$A:B:C=1:2:3$，则$a:b:c$等于（　　）