参考答案

正确答案：B
B。【解析】修改表结构的命令是ALTERTABLE，该命令有3种格式。①ALTERTABLETableNamelADDIALTER[COLUMN]FieldNamelFieldType[(nFieldwidth[nPrecismn])][NULLINOTNULL][CHECKlExpressionl[ERRORcMessageTextL1][DEFAULTeExpressionl][PRIMARYKEYKEYlUNIQUE3EREFERENC：ESTableName2ETAGTagName]]⑦ALTERTABLETableNamelALTERECOLUMN]FieldName2[NULLlNOTNULL]ESETDELAULTeExpression2][SETCHECKlExpression2[ERRORcMessageText2][DROPDEFAULT][DROPCHECK]③ALTERTABLETableNamet[DROP[COLIJMN]FieldName3][SETCHECKlExpression3[ERRORcMessageText3]][DROPCHECK][ADDPRIMARYKEYeExpression3TAGTagName2[FOR1Expression4]][DROPPRIMARYKEY)[ADDUNIQUEeExpression4[TAGTagName3[FOR1ExpressionS]]]EDRPUNIQUETAGTagName4][ADDFOREIGNKEY[eExpression5]TAGTagName4[FOR1Expression6]REFERENCESTableName2[TAGTagName5]][DROPFOREIGNKEYTAGTagName6[SAVE]][RENAMECOLUMNFieldName4TOFieldName5]