(30 points) [relational calculus] write queries a, b, d, and g of textbook exercise 8.18 in tuple relational calculus. see the following additional clarifications/requirements:  for the exercise a, retrieve the number of copies of the book.  for the exercise b, retrieve the library branch id and the number of copies of the book.  for the exercise d, assume a system function $sysdate() which returns today’s date.  for the convenience of grading, use the following tuple variables for each relation: b for book, a for book_authors, p for publisher, c for book_copies, l for book_loans, h for library_branch, and r for borrower. use the minimum number of relations, that is, do not use a relation that we can do without to express the query. note that tuple variables that are retrieved as query result should not be bound (e. g., quantified) in the predicate formula.