Consider the following: Mysql> EXPLAIN SELECT * FROM City WHERE Name = ‘Jacksonville’ AND CountryCode = ‘USA’ \G ******************************** 1. row ******************************** Id: 1 Select_type: SIMPLE Table: City Type: ref Possible_keys: name_country_index Key: name_country_index Ref: const, const Rows: 1 Extra: Using where Which statement best describes the meaning of the value for the key_len column? A. It shows the total size of the index row. B. It shows how many columns in the index are examined. C. It shows the number of characters indexed in the key. D. It shows how many bytes will be used from each index row. 答案:D