Explanation:
Good hit ratios in the library cache imply acceptable hit ratios in the dictionary cache if data dictionary information is held in memory longer than library cache data.
Incorrect Answers:

1: You need to monitor library cache usage any way to improve its performance.
2: There is no direct relation between time of data dictionary information holding and database buffer tuning.
3: The library cache and dictionary cache are related very closely, so it's impossible to tune them independently.
5: The dictionary cache will not imply acceptable hit ratios on the database buffer cache: they do not correlate directly.
Oracle 8, DBA Certification Exam Guide, Jason S. Couchman, p. 849-855. Chapter 18: Tuning Memory and Operating System Use

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