Isotope is the study and application of stable and radioactive isotopes to geological processes and their time scales.
From what I remember, c is the correct answer.
Lagos ,Nigeria kinda cool
One way would be to limit who can receive a passport or visa to travel to another country and another would be at the entrance to the country they may not be admitted for example due to inadequate funds to live. In my case, when I travelled by bus from Costa Rica to Panama, the Panamanian authorities required that I either have a ticket out of the country or a return ticket back to San Jose, Costa Rica.
