The following questions present a sentence, part of which or all of which is underlined. Beneath the sentence, you will find five ways of phrasing the underlined part. The first of these repeats the original; the other four are different. If you think the original is best, choose the first answer; otherwise choose one of the others.
These questions test correctness and effectiveness of expression. In choosing your answer, follow the requirements of standard written English; that is, pay attention to grammar, choice of words, and sentence construction. Choose the answer that produces the most effective sentence; this answer should be clear and exact, without awkwardness, ambiguity, redundancy, or grammatical error.
Because a rockslide had blocked much of the passage, the spelunking team was not able of exploring the till extent of the cave.（）
A. the spelunking team was not able of exploring the full extent of
B. the full extent was not able to be explored by the spelunking team of
C. the spelunking team was unable in exploring, to its full extent,
D. therefore the spelunking team was unable to explore the full extent of
E. the spelunking team was unable to explore the full extent of