单选题 ______is a phenomenon that in some speech communities two languages exist side by side with each having a different role to play; and language switching occurs when the situation changes.
【正确答案】 A
【答案解析】解析：考查双语现象。在某些语言社区中，两种语言同时存在，各自扮演不同的角色，当场合变换时，两种语言之间发生转换，这被称为双语现象。