You want to start monitoring statistics on the distribution of storage engines that are being used and the average sizes of tables in the various databases. Some details are as follows: The Mysql instance has 400 databases. Each database on an average consists of 25-50 tables. You use the query: SELECT TABLE_SCHEMA, 'ENGINE', COUNT (*), SUM(data_length) total_size FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_TYPE= 'BASE TABLE' GROUP BY TABLE_SCHEMA, 'ENGINE' ; Why is this query slow to execute?（）

• A、Collecting information requires large numbers of locks on various INFORMATION_SCHEMA tables
• B、Aggregating details from various storage engine caches for the final output is time consuming
• C、Collecting information requires various disk-level operations and is time consuming
• D、Counting and summarizing all table pages in the InnoDB shared tablespace is time consuming