请教几道黑带考试题

1.When σ=10, what sample size is need to specify a 95% confidence interval of ± units from the mean?
(A).7 (B). 11 (C). 32 (D).43

1. A six sigma project to reduce billing statement expenses has shown the need to hire two additional mailroom clerks. Based on this information, which of the following metrics should be used to measure the finacial benfits of the project

(A). Cost of poor quality (B). Return on investment (C). Net present value (D). Internal rate of return

1. A company's acount payable department is trying to reduce the time between receipt and payment of invoices. If the team has just complete a flowchart of the process and identified the critical steps, which of the following tools should be used next

(A). Fishbone diagram (B). Scatter diagram (C). Box and whisker plot (D), Histogram

上面三道题请高手帮忙做一下，并说明原因。跪谢！