Which four statements correctly describe functions that are available in SQL?（）

• A、INSTR returns the numeric position of a named character.
• B、NVL2 returns the first non-null expression in the expression list.
• C、TRUNCATE rounds the column, expression, or value to n decimal places.
• D、DECODE translates an expression after comparing it to each search value.
• E、TRIM trims the heading of trailing characters (or both) from a character string.
• F、NVL compares two expressions and returns null if they are equal, or the first expression of they are not equal.
• G、NULLIF compares twp expressions and returns null if they are equal, or the first expression if they are not equal.