Through thorough investigation, Dr. John Snow was finally able to prove that the outbreak of Cholera _____ people’s drinking of polluted water.     [ ]

A. was close linked with  

B. closely linked to  

C. was close connected with

D. was closely linked with

变频器的控制方式有（）。

A、开关量控制变频器

B、数字量控制变频器

C、电流控制变频器

D、电压控制变频器