Sometimes, two varieties of a language exist side by side throughout the community, with each having a definite role to play. This phenomenon is ______ .

A．can better the traffic unconditionally
B．calls for a driver’s cool-headed and good-tempered personality
C．can be found after the modem traffic conditions stand up
D．are acknowledged by most drivers

A．use of substances may vary from time to time
B．abuse of certain substances is harmful for health
C．the attack of September 11th has left incurable harm to people’s mental health
D．terrorist attack increases anxiety and sense of insecurity among residents