动物的“学习”能力主要与动物的什么结构有关（　　）

Dear Simon,

    How are you? Last week I ____ to Xi'an. It's in the northwest of China. I went there ____ plane. The plane took us to the ____. First, I was a little  ____. Then I looked out of the window.

The plane was above the ____. I ____ some mountains below. I ____ lunch on the plane. Later I arrived in Xi'an. I ____ many interesting places. The trip was great!

Love,

Daming

二氧化碳在空气中只占____，但它是绿色植物进行____作用中并不可少的原料之一。通常状况下，二氧化碳是一种____色 ____味的气体，其密度比空气的密度____ ，____溶于水。在一定条件下，二氧化碳变成固体，俗称 ____。