The engineers believed that the roller coaster was safe to ride again; , they decided to keep it closed until further test trials could be completed. Which transition best completes the sentence?

"but"

Explanation:

The best and only transition that works for this sentence would be "but". This states that even though the ride was safe the engineers still decided to go against the logical next step and maintain the ride closed until they could make certain that it was completely safe. If the second part of the sentence was a logical conclusion the transition could be "so" or "and". But that is not the case so this would be the best transition for the sentence.