Each of them accepts an internal request only if it is not executing an other request. The lift control device can register several incoming request at the same time. Internal requests are fulfilled by the lift, where they were entered. The control device selects a suitable free lift to fulfill each external request.
All lifts stop at floor zero.
Consider the case in 3, and suppose that there can be several pending internal requests from each lift, not just one. All internal requests are accepted and registered, no matter whether a lift is free or not. During the next tick, a lift could go one floor up or down, or remain at the same floor. At each tick the program should display information about the location of each lift. The lifts are large enough and cannot be overloaded.