The valid CREATE TABLE commands are A and D. Let's analyze why these are correct: A. This statement is valid because it uses BOOK_SEQ.NEXTVAL to automatically generate a unique primary key value for each row in the table. The SYSDATE function for the start_date and end_date columns is also valid. B. This statement is invalid because it tries to use a logical expression (end_date >= start_date) in the DEFAULT clause, which is not allowed. C. This statement is invalid because it uses BOOK_SEQ.CURRVAL for the bk_id column. CURRVAL might not have a value when the sequence is first used, causing issues. Additionally, it has the NOT NULL constraint before the DEFAULT clause, which is incorrect syntax. D. This statement is valid because it correctly defines the bk_id as NOT NULL and PRIMARY KEY, which is indeed redundant since PRIMARY KEY implies NOT NULL, but it is still syntactically valid. The use of the SYSDATE function for start_date and end_date is also valid. E. This statement is invalid because it references another column (start_date) in the DEFAULT clause, which is not allowed.