(a) 



5.00- 



ro 4.80- 



Lower quality 



Higher quality 



7 present 



(b) 



5.20- 



4.60- 



(c) 



5.60- 



5.50- 



03 5.40- 



c 



CO 



CD 



2 5.30- 



ro 



c 



O) 4.20- 



cc 



5 4.10- 

 5.00- 

 4.90- 



7 present 



7 absent 



Lower quality 

 Higher quality 



7 present 



16 



