Which one of the following is true of "within group sum of squares" for each number of clusters in the chart? it is the cohesiveness of all clusters. it is the distance errors for the first cluster. it is the ratio of within cluster errors to errors between clusters. it is the difference between distances of cluster centers. According the the plot, what would be the optimal number of clusters? 3 6 8 13 Which one of the following best describes the situation where the wss would be zero. each data point is in its own cluster. all data is in one cluster. the data are in the optimal clustering. the distance metric is city block