Which of the words doesn't belong in the group? Write it down. (p.68/1a)

Gap-fill exercise

  
Fill in all the gaps, then press "Check" to check your answers.
wood -- plastic -- paper -- glass --->
oil -- coal -- wind -- gas --->
waste -- recycle -- reduce -- save --->
pollute -- destroy -- burn -- protect --->
dangerous -- safe -- poisonous -- threatening --->