属于生物基本特征的是（  ）

    I'm a middle school student from America. I live far away from my school. So I often go to school by bus. It takes me about twenty minutes from my home to school.

    But at my school and other schools in our town, there is a "Walk to School Day". It is on Wednesday every week. On that day, parents send their kids to a bus stop. There are many bus stops in our town. At each bus stop, two adults lead us to walk to school together.

    "Walk to School Day" stops us taking cars or buses. That is good for the air and we can get exercise too. On the way to school, we talk to each other. We can also make new friends. I like "Walk to School Day". I walk about five kilometers with other students on that day. Our school will give some students beautiful backpacks if they do well. And I got one last year.

在平整地面上有一层厚度均匀的积雪，小明用力向下踩，形成了一个下凹的脚印，如图所示．脚印下的雪由于受外力挤压可近似看成冰层，利用冰的密度，只要测量下列哪组物理量，就可以估测出积雪的密度？（　　）

诗句“两岸猿声啼不住，轻舟已过万重山”中，描写轻舟运动时所选的参照物是（）