To solve a problem with input size N by divide and conquer algorithm, if the conquer step takes O(1) extra work to form the solution from the sub-solutions, then among the following four dividing methods, which is the best. ( ) A. divide into 3 sub-problems of equal complexity N/2 B. divide into 2 sub-problems of equal complexity N/3 C. divide into 3 sub-problems of equal complexity N/3 D. divide into 2 sub-problems of equal complexity N/2