正常情况下，当人体局部组织活动增加时，代谢产物增加，此时该组织中的（    ）

阅读下面材料，在空白处填入适当的内容（1个单词）或括号内单词的正确形式。

      I was nine years old when I learnt to ride a bike. The bike was a birthday present from my uncle. You can imagine how  ____(excite)I was when I saw the bike, and I begged my uncle to teach me how  ____(ride)it right away. However, before I got on the bike, I felt  ____(extreme)nervous and I was afraid that I would fall off and hurt ____(me). My uncle held onto the seat and helpedme to get  ____the bike. While I was riding, he wasrunning along beside me, holding the seat so I would not fall over. I was so ____(grate) to him for his help.

    As I was practicing, I became more and more confident. Then, Iheard my uncle shouting, “You are riding it by yourself now!” I was bothexcited and scared. I was really riding by myself, but  ____if I fell off? Could I use the brakesto stop? As I was wondering about this, my uncle ran after me and got hold ofthe bike. I  ____(slow) down and came to a stop. Myheart was still beating fast when I got off the bike. ____ (late) that day, having learnt how to get onthe bike and get off it, I felt like I was walking on air! I am still thankfulto my uncle for teaching me to ride a bike on my ____ (nine) birthday.

从进化角度来看，下列符合生物进化规律的一项是（    ）

滑板运动是青少年喜爱的运动之一，如图所示．用脚向后蹬地，滑板车会沿地面向前加速运动，这说明力的作用是　____ 　的；滑行时，它会受到　____ 　摩擦力作用（选填“滑动”、“滚动”或“静”）；如果不再蹬地，滑板车的速度会越来越慢，最后停下来，这表明力可以改变物体的____ 

在化学反应A＋B===C＋D中，已知12gA和18gB恰好完成反应生成6gC，若实际反应时生成了8gD，则参加反应的B的质量是（   ）