A．Because the smokers fad became a habit that had to be fed.
B．Because more and more people were involved in planting tobacco and selling it.
C．Because people believed that smoking could develop one’s brain.
D．Because smoking spread to all parts of the world.

A．Some birds can only use the sun to guide them.
B．Some birds can only use the stars to guide them.
C．When it is too dark to see anything, birds still can find their direction.
D．Any change in the position of the make-believe stars would cause a change in the direction of the birds flights.