单选题 《复合题被拆开情况》 Why can you look at an object in the real world and see it as a three-dimensional (三维的) object, but if you see that same object on a television, it looks flat? What’s going on, and how does 3-D technology get around the problem? It all has to do with the way we see objects. When the light that travels to an object is sent back to our eyes, our brains understand the light and make up a picture in our minds. When an object is far away, the light travelling to one eye is parallel (平行) with the light travelling to the other eye. But as an object gets closer, the lines are no longer parallel—they move toward the same point where they join. You can see this effect if you try to look at something right in front of your nose—you’ll get a lovely cross-eyed expression. When you look at an object, your brain measures the effort your eyes make to focus on the object as well as how much they had to gather the light to the same point. Together, this information allows you to guess how far away the object is. If your eyes had to gather the light with quite a big effort, then it stands to reason that the object is close to you. The secret to 3-D television and films is that by wearing a special pair of glasses and by showing each eye the same picture in two different places, you can make your brain think that the flat picture you’re seeing has depth. But this actually doesn’t match up the way we see real objects. While your eyes may gather two images (形象) that seem to be one object right in front of you, they’re actually focusing on pictures further away. This is why your eyes get very tired if you try to watch too many 3-D movies in one sitting.《问题》：How does our brain judge the distance between us and an object?