单项选择题A．Because marketing groups have collected consumers’ information for years.B．Because marketing groups can send messages around the world for free.C．Because people keep down junk mail to a manageable level.D．Because it is cheaper to mail packages to large groups of people.

单项选择题A．They eat protein.B．They never move from one location.C．They cause erosion of rocks.D．They will never be useful to people.