The sentence below is from an exam held nationwide in Turkey in 2005. The answer is A, but why "imply" is wrong? doesn't it have a sense meaning require?

Marathon-training schedules range from four to six months and they all ---- considerable


A) require B) reduce C) imply D) combine E) improve
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Comments  (Page 3) 
Dear Diamondrd
May I also suggest that you look up require and imply in a variety of dictionaries, particulary those with comprehensive analysis of the various meanings, and also try to see/hear the words used in various contexts. This may help you get a broader/better understanding of the meaning and usage of these words.
There are some instances where either word may be used, without changing the overall meaning, but in most situations only one of them is correct.
Best wishes