The manager of a public swimming pool wants to compare the effectiveness of two laundry detergents, Detergent A and Detergent B, in cleaning the towels that are used daily. As each dirty towel is turned in, it is placed into the only washing machine on the premises. When the washing machine contains 20 towels, the manager flips a coin to determine whether Detergent A or Detergent B will be used for that load. The cleanliness of the load of towels is rated on a scale of 1 to 10 by a person who does not know which detergent was used. The manager continues this experiment for many days. Which of the following best describes the manager's study? 
选项为:A:A completely randomized design  B:A randomized block design with Detergent A and Detergent B as blocks  C:A randomized block design with the washing machine as the block  D:A matched-pairs design with Detergent A and Detergent B as the pair  E:An observational study  请回答其中一个选项.