设有字符串变量String A ＝“”，B＝“MULE”，C＝“OLD”，D＝“MY” ; 请计算下列表达式 （3个答案本身不要出现空格，答案之间用空格分开） Assume that there is a string variable String A = "", B = "MULE", C = "OLD", D = "MY"; Please calculate the following expression: (1) D+C+B (2) B.substr(3，2) (3) A.strlength()