用方框中所给单词的适当形式填空，使对话通顺、正确。每词仅用一次（每空一词）。

Marie：Roy and Carla, what are your ideas for solving these pollution problems？

Roy：To cut down air pollution，we should take the bus or subway instead of   ____ 

Carla：Yeah, or ride a bike. There are other advantages of bike riding. It's good for health and it doesn't____   much！

Marie：Great ideas！What about waste pollution?

Carla：Mmm, I thank simple things like bringing a bag to go shopping can help. I started doing that a year ago.

Roy：Me，too. Also I never take wooden chopsticks or plastic____ when I buy takeaway food. I use the ones at home．

Carla：And remember to throw rubbish in the bins and keep____ places clean and beautiful for everyone·

Marie：So together, our actions can make a____ and lead to a better future！

在用插针法测定玻璃砖折射率的实验中，甲、乙、丙三位同学在纸上画出的界面ab、cd与玻璃砖位置的关系分别如图①、②和③所示，其中甲、丙同学用的是矩形玻璃砖，乙同学用的是梯形玻璃砖．他们的其他操作均正确，且均以ab、cd为界面画光路图．

则甲同学测得的折射率与真实值相比____（填“偏大”、“偏小”或“不变”），乙同学测得的折射率与真实值相比____（填“偏大”、“偏小”或“不变”），丙同学测得的折射率与真实、值相比____．