The Colosseum at the time of its completion: A. Was the only Roman structure to use concrete as its foundation B. Was the largest arena ever created for entertainment C. Was the first structure designed for gladiatorial games D. Was the largest structure ever to be built by mankind

B. Was the largest arena ever created for entertainment.

Explanation:

The Colosseum was commissioned by emperor Vespasian around 70-72 AD, it was intended to be a gift for the people of Rome. Vespasian´s son, emperor Titus, inaugurated it with 100 days of games and spectacles. These included gladiator duels and wild beast fights. It was used for four centuries, after that it was neglected. The Colosseum is a symbol of Rome and its tempestuous history.