Passenger 1 and passenger 2 are booking separately on an airline website for the last available seat on a flight. Passenger 1 presses the enter key a few seconds before passenger 2, thus locking out passenger 2 and obtaining the last seat. This locking is a form of which of the following types of control?

a. Concurrent update control.

b. Compensating control.

c. Data-entry control.

d. Operational data control.