It is expected that the world's population will reach ______ by the end of the century.

A．to make the rain fall in the sea before it reaches the coast
B．to make the rain clouds
C．to cause monsoon in the sea
D．to bring heavy rain

A．the rain is caused by water-vapor
B．the higher the air rises, the higher the temperature is
C．the higher the air rises, the lower the temperature is
D．the clouds rise, and the water drops become smaller