题目内容：
Questions refer to the following article.Now let us look at how we read. When we read a printed text, our eyes move across a page in a short, jerky movement. We recognize words usually when our eyes are still when they fixate.
Each time they fixate, we see a group of words. This is known as the recognition span or the visual span. The length of time for which the eyes stop the duration of the fixation-varies considerably from person to person. It also varies within any one person according to his purpose in reading and his familiarity with the text. Furthermore, it can be affected by such factors as lighting and tiredness.
Unfortunately, in the past, many reading improvement courses have concentrated too much on how our eyes move across the printed page. As a result of this misleading emphasison the purely visual aspects of reading, numerous exercises have been devised to train the eyes to see more words at one fixation. For instance, in some exercises, words are flashed on to a screen for, say, a tenth or a twentieth of a second. One of the exercises has required students to fix their eyes on some central point, taking in the words on either side. Such word patterns are often constructed in the shape of rather steep pyramids so the reader takes in more and more words at each successive fixation. All these exercises are very clever, but it's one thing to improve a person's ability to see words and quite another thing to improve his ability to read a text efficiently. Reading requires the ability to understand the relationship between words. Consequently, for these reasons, many experts have now begun to question the usefulness of eye training, especially since any approach which trains a person to read isolated words and phrases would seem unlikely to help him in reading a continuous text.
The time of the recognition span can be affected by the following facts except____________. A．one's familiarity with the text
B．one's purpose in reading
C．the length of a group of words
D．lighting and tiredness
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