All the task          ahead of time, they decided to go on holiday for a week.[       ]

A. had been finished      

B. were finished  

C. having been finished  

D. been finished

为调节离心泵的流量，可改变出口阀开度，或改变叶轮转速。两者相比，哪种方法经济上更合理一些？（）

A.不好说

B.两种方法经济性相差不大

C.改变出口阀开度

D.改变叶轮转速