Which of the following is a valid command? a. CREATE INDEX book_profit_idx ON (retail-cost) WHERE (retail-cost) > 10; b. CREATE INDEX book_profit_idx ON (retail-cost); c. CREATE FUNCTION INDEX...

Which of the following is a valid command?
a. CREATE INDEX book_profit_idx
ON (retail-cost) WHERE (retail-cost) > 10;
b. CREATE INDEX book_profit_idx
ON (retail-cost);
c. CREATE FUNCTION INDEX book_profit_idx
ON books WHERE (retail-cost) > 10;
d. both a and c
e. none of the above