[简答题]辨认主题句
Read the following paragraph and underline the topic sentence.    When a camera flash is used in a low-light environment, human eyes may appear red in the finished photograph. What is known as "red-eye" is the result of light from the flash reflecting off the pupils (瞳孔) of the eyes. The phenomenon of red-eye can be avoided by using the red-eye reduction feature found on many single-lens-reflex cameras. This feature triggers a lamp which shines a small light directly into human eyes. When this happens, the diameter of the pupils is reduced, thus tightening the opening in the iris (虹膜). Since a smaller pupil means a smaller host for the reflection, the chances of red-eye occurring are greatly reduced.