The underlined words "an activity" in the context refer to ______

A．In the 1970s some writers have avoided showing violence and began to explore the criminal mind, and returned to the old style of hooking the reader by slowly revealing a series of clues.
B．In the 1980s' crime novels, the increasing numbers of male investigators were smart and capable of dealing with dangerous situations.
C．MacDonald's stories about salvage expert Travis McGee shed light on the corruptions of modem life.
D．In the 1970s many American writers of detective fiction began to focus, at least in part, on their detective's personal lif

A．kidnapping has become the most profitable business in many parts of the world
B．there is a worldwide increase in kidnappings and many groups have make profits in kidnaps
C．by kidnapping the kidnappers have established their corporations in many parts of the world and earn big money each year
D．in many parts of the world more people have become kidnappers