A．while pushing asteroids off course nuclear weapons would destroy the world
B．asteroids racing across the night sky are likely to hit Earth in the near future
C．the worry about asteroids can be left to future generations since it is unlikely to happen in our lifetime
D．workable solutions still have to be found to prevent a collision of asteroids with Earth

A．the capacity to generate electricity is the distinctive characteristic of the fish
B．the current travels in an upward direction from the positive side to negative side in torpedo’’s electric cells
C．some fish can produce enough electricity to drive a number of electric motors
D．the torpedo’’ s electric cells have a shape with six sides