16. (8 p.) The semijoin of relations S and R, denoted S R, is the set of tuples of S that agree with at least one tuple of R on all attributes that are common to the schemas of S and R. Note that the schema of S x R is the same as the schema of S. Consider the database schema given in Problem 10. Give four different expressions of relational algebra that are equivalent to (i.e. produce a same result as) Product x PC Problem 10 Consider the following relations. Product (model, maker, type) PC(model, speed, ram, hd, price) Laptop (model, speed, ram, hd, screen, price) Printer(model, color, type, price) The Product relation gives the model number, the manufacturer and the type (pc, laptop or printer) of various products. We assume for convenience that model numbers are unique over all manufactures and product types. The PC relation gives for each model number that is a PC the speed (of the processor, in gigahertz), the amount of RAM (in megabytes), the size of the hard disk (in gigabytes), and the price. The Laptop relation is similar, except that the screen size (in inches) is also included. The Printer relation records for each printer model whether the printer produces color output (1, if so, and 0 otherwise), the printer type (laser, ink-jet, or other), and the price