 Which term best describes the ease of space travel within the context of the passage?
But since there were ice-caps there would be temperate regions. In short, the ice-cap proved that a man could endure the air and temperature conditions he would find. With clearance-papers in order, they could land unquestioned at any other space-port and take off again—provided the other space-port was one they had clearance for. Without rigid control of space-travel, any criminal anywhere could escape the consequences of any crime simply by buying a ticket to another world. Wherever the yacht landed, such a disparity between its documents and its crew would spark an investigation. But space-travel regulations are especially designed to prevent such escapes. Interplanetary drive is rugged and sure, but it does not respond to fine adjustment.
 strict
 complex  evolving  strict  flexible