A． install many pollution filters
B． spray a new material on engines
C． reduce the use of buses
D． cut down the use of fuel

A． A new fuel for buse
B． The causes of air pollutio
C． A way to improve fuel efficiency in buse
D． Careers in environmental engineerin

A． He is always laughing at other student
B． He was constantly talking with other students in clas
C． He disturbed the teacher by asking question
D． He is always showing off in the classroo