A. A single ADR can support multiple ADR homes for different database instances.
B. The alert files are stored in XML file format in the TRACE directory of each ADR home.
C. If the environmental variable ORACLE_BASE is set,then DIAGNOSTIC_DEST is set to $ORACLE_BASE.
D.The BACKGROUND_DUMP_DEST initialization parameter overrides the DIAGNOSTIC_DEST initialization parameter for the location of the alert log file.
答案：（A、C）
解析：
参考《Concepts Concepts》 13.6.1.2
这个参数被Oracle 11g新引入的诊断架构所忽略,这个架构会将trace and core file放到由DIAGNOSTIC_DEST初始化参数所控制的位置上