If xi=total number of nonconformities in a sample of ni inspection units, and there are N samples of different sample size, and x varies according to a Poisson distribution, what will be the value of center line of the u-chart?

• A. ( bar{u} = frac{ sum_{i=1}^N frac{2x_i}{n_i}}{N} )
• B. ( bar{u} = frac{ sum_{i=1}^N frac{x_i}{2n_i}}{2N} )
• C. ( bar{u} = frac{ sum_{i=1}^N frac{x_i}{n_i}}{N} )
• D. ( bar{u} = frac{ sum_{i=1}^N frac{x_i}{n_i}}{2N} )