An xSeries specialist monitors a customer’s server that has poor performance.  The customer is running an x365 with a ServeRAID 6i controller with write-back cache turned on and ten 15K rpm drives in a RAID-5 array.  It is a mission-critical server and must have a redundant storage system.  Performance tests indicate that the Average Disk sec/Transfer exceeds 20 ms.  Which of the following is the lowest-cost method to increase the throughput？（）

• A、Disable the ServRAID cache
• B、Change the RAID level to 10
• C、Change the cache level to Write Through
• D、Add five more drives to the array to increase the number of read/write heads