Compact Computers is a small computer assembly company. Any customer currently has the following choices for a PC: (i) 800 MHz processor, 40 GB HDD, 128 MB RAM (ii) 1 GHz processor, 60 GB HDD, 256 MB RAM (iii) 1.2 GHz processor, 80 GB HDD, 512 MB RAM The use of what design pattern would ensure that only the legal combinations could be sold?