Complex sentence: In order to build level 0 of the "k"-d tree, "each" of these "k" -1 arrays must be partitioned by the "x"-splitting plane to produce two subarrays.
Split into simpler sentences: In order to build level 0 of the "k"-d tree, the "k" -1 other arrays must be considered. Therefore, "each" of these "k" -1 arrays must be partitioned by the "x"-splitting plane to produce two subarrays.