We know that the adverb 'gradually' modifies 'spread' . Thus, 'gradually' should come after 'spread' and not before 'thought' But in B the reverse is the case and thus, option B is incorrect Now, the adverbial phrase, 'through an orderly process' also modifies 'spread' and should be placed it after that. This is not the case either in C or D In Both C and D it modifies 'thought' which is incorrect. Thus, only option A is correct.