根据以下材料，回答
As with spoken language, writtenlanguage is always used for a purpose. People read a text 11 they think that it will enable them tofind answers to questions that they are interested in answering. People writeto express an 12 or to give information to particularreaders. There are, of course, many different purposes for reading and writingand different purposes will 13 different reading and writing styles.
In general, written language isstructurally more "correct" than spoken language. It has clear wordand sentence 14 and its information is more denselypacked. More is said in 15 words. However, written language alsocontains both structural and contextual redundancy and this can help readers to 16 the text.
Written language is often structurally more 17 than spoken language. This is becausewhen people write they have 18 to think about what they want to writeand are able to 19 to what they have written and revise itas often as they wish. This greater, structural complexity is one factor that maymake a text 20 to understand.
When people read in their first language,they do not usually read every word in the text. Readers 21 their eyes across and down the textstopping at groups of words (fixations) to check for meaning. The speed withwhich people read 22 their purpose for reading and on how 23 a range of possible meanings their brainhas to choose from at every fixation. 24 readers use the structuraland contextual redundancy of the language, their 25 of what they have already read and thegeneral knowledge they already have to 26 what will come next, and so 27 the number of possible choices that thebrain has to consider at any fixation.
Both writers, when choosing how to expresstheir 28 meaning for the audience that they have inmind, 29 readers, when interpreting writers′meaning, rely not only on their linguistic knowledge, but 30 their general knowledge of the context inwhich they are reading
A.why
B.because
C.after
D.before