It is an ______ which may be brown, green or black in color.

A．have too many boats on them
B．are overcrowded on the beaches
C．are polluted by wastes
D．are no longer attractive

A．The development of tourism has brought about environmental problems.
B．Tourism should not develop at the cost of the environmental destruction.
C．Measures taken to protect the environment prove to be ineffective.
D．Tourism is developing for the sole purpose of making money.