（單選）In a traditional merge sort, two sorted sub-arrays are combined to form a single, fully sorted array. This is referred to as a 2-way merge. The problem at hand is to extend this concept by merging $N$ sorted arrays of integers, where $N$ and $M$ are given integers representing the number of arrays and the number of integers in each array, respectively. Please choose the correct worst-case time complexity of this N-way merge.