Katherine: Linda! I haven"t seen you for ages. How are you Linda: Fine. And you Katherine: Pretty good. How"s Frank Linda: Oh, don"t you know We got divorced two years ago. Katherine: ______.

A．A group of people may solve problems more effectively than an individual.
B．The efficiency of a group is worst in the world.
C．A group will not be efficient in dealing with national policies
D．Effective leaders often solve the problems themselves

A．at
B．in
C．of
D．by