150*75=11250.
r1 and r2 have C in common.C is primary key in r2,that means C in r1 can repeat.
Now r1 natural join r2 will contain at max 150 tuples.
The table obtained after r1 natural join r2 will have nothing in common with r3.In natural join if no attribute is common then no of tuples in both table will be multiplied. Therefore 150*75 = 11250
