The correct answer is D.
The car, railway and airline industries were astonishingly developed during the first half of the 20th century. At first, only upper classes and richer citizens could pay the price to use these means of transport, but not much time later they became affordable for everybody and this is the main reason why leisure travel appeared.
On the other hand, internet was invented in the 1970s and it was not massively used in households until the 1990s and 2000s. Online travel websites date back from even later.
