Reading experts have been interested for a long time in discovering what makes reading material difficult. A great deal of 【B1】_

Reading experts have been interested for a long time in discovering what makes reading material difficult. A great deal of (26) research has been done on the topic. And now most experts (27) agree that there are two major (28) factors in reading difficulty.  One is the (29) difficulty of the words found in the (30) selection. If they are long, (31) unknown, or technical words, the material is more difficult. The other major factor is the (32) complexity of the sentence. (33) Short, simple sentences are easier to understand.  Long, involved sentences with many dependent clauses and other subparts are more difficult.
     Some experts have developed ways of measuring the readability or difficulty of material. (34) Word difficulty is usually measured by word length or word frequency, since some words are used in our language many more times than other words. Sentence complexity is usually measured by using the average number of words in the sentence. These two factors are effective to measure word difficulty and sentence complexity.
     The usual purpose of measuring difficulty or readability is to be able to provide suitable reading material to children at a certain level. (35) As a result, readability is usually expressed at a grade level. It is supposed to identify the grade level of the person who can successfully read and understand it.  For example, material that is at seventh grade on a readability measure would be much too difficult for most children in the third grade. In this way, the material provided can match their reading ability.