EMPLOYEES表结构如下：
EMPLOYEE_IDNUMBERNOT NULL, PRIMARY KEY
LAST_NAMEVARCHAR2(30)
FIRST_NAMEVARCHAR2(30)
JOB_IDNUMBER
SALNUMBER
MGR_ID字段的数据类型为NUMBER，参照EMPLOYEE_ID列和DEPARTMENT_ID列（NUMBER）。现在需要在FIRST_NAME和LAST_NMAE字段上建立名为NAME_IDX的索引，下列哪个SQL语句能执行这项任务：( )
CREATE INDEX NAME_IDX(first_name, last_name);
CREATE INDEX NAME_IDX(first_name AND last_name);
CREATE INDEX NAME_IDX ON employees(first_name, last_name);
CREATE INDEX NAME_IDX FOR employees(first_name, last_name);
