The author thinks that the conclusions of any research about people's state of mind are______.

A．They are taking the place of high-flying jets.
B．They are used for rescue work only.
C．They have been widely used for various purposes.
D．They are now used for commercial purposes.

A．For extremely high altitude flights.
B．For urgent mission to places inaccessible to other kinds of craft.
C．For rescue work.
D．For high-speed transportation.