In the past twelve months the research members ______ the effectiveness of many materials in a home building.

A．had studied

B．would study

C．studied

D．have studied

当客户呼入10086投诉客户代表服务态度时，是否应向客户了解清楚客户所投诉的客户代表态度如何恶劣，还是应向客户了解其真正的需求？