Which of these is another form of the cumulative sum value plotted on the cumulative sum chart?

• A. (C_i=(x_i- )+ sum_{j=1}^{i-1} (x_j+ ) )
• B. (C_i=(x_i- )+C_{i-1} )
• C. (C_i=(x_i- )+ sum_{j=1}^{i-1} (x_j+2 ) )
• D. (C_i=(x_i- )-C_{i-1} )