In the early 1800s, british engineer richard trevithick invented the first railway steam locomotive. by 1850, britain had more than 6,000 miles of railroad track. these two events are an example of which type of relationship?

Causation

Explanation:

A casual relation is one that represents two events,where the second event was caused by the first event.In case of the text shown in the question, the miles of railroad track, were built because previously the first railway steam locomotive was created.

i believe the correct answer is option b. most eastern european nations joined nato.

though i could be wrong, option d. the eastern european nations were granted complete independence, would be another choice.

hope this ! : )

i would say your answer is a)

sorry if im wrong