You are given the following database structure, pay attention to all keys as it may affect your answer.

Please answer the following requests in Relational algebra:

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Assume the same database as in Homeworks 1 and 2: - BOOK(isbn, title, num-pages, publisher, year, genre, price, aut-id). The first attribute is the primary key; the last one is a foreign key to AUTHOR. - CUSTOMER (customer-id, name, address, city, state, zip, age). The first attribute is the primary key. - SALES(isbn, custid, num-copies, day, month, year, total). The first attribute is a foreign key to BOOK and the second a foreign key to CUSTOMER. The key of this table is the two foreign keys together. - AUTHOR (author-id, name, dob, nationality, address, city, state, zip). The first attribute is the primary key. Attribute dob stands for 'date of birth'. Attribute nationality has values like 'British', 'Australian', etc. Attribute genre has values like 'sci-fi', 'romance', etc. Note that this schema assumes that each book has one and only one author, but it can be sold to different customers at different times. A customer can buy several copies of several books. 1. Write the following queries in Relational Algebra: (a) List the name and address of customers who have bought either a sci-fi book or a romance book (recall that 'sci-fi' and 'romance' are values of genre). (b) List the name and address of customers who have bought both a sci-fi book and a romance book (recall that 'sci-fi' and 'romance' are values of genre). (c) List the name and address of customers who have never bought a sci-fi book. (d) List the name and address of customers who have bought at least two different books (not two copies of a book)