A plasmid contains genes for ampicillin resistance and tetracycline resistance and has single sites for restriction sites BglII, PstI and EcoRI. When genes are inserted into the PstI site, transformants are resistant only to ampicillin. In which region is the site for PstI located?

a. within the ampicillin gene

b. within the tetracycline gene

c. between the ampicillin and tetracycline genes.