一道语法题求助！

Construction of the Roman Colosseum, which was officially known as the Flavian Amphitheater, began in A.D. 69, during the reign of Vespasian, was completed a decade later, during the reign of Titus, who opened the Colosseum with a one-hundred-day cycle of religious pageants, gladiatorial games, and spectacles.

1. which was officially known as the Flavian Amphitheater, began in A.D. 69, during the reign of Vespasian,
2. officially known as the Flavian Amphitheater, begun in A.D. 69, during the reign of Vespasian, and
3. which was officially known as the Flavian Amphitheater, began in A.D. 69, during the reign of Vespasian, and
4. officially known as the Flavian Amphitheater and begun in A.D. 69, during the reign of Vespasian it
5. officially known as the Flavian Amphitheater, which was begun in A.D. 69, during the reign of Vespasian, and

这道题答案是A，为什么B不能选，我觉得B更简洁。是不是因为A中WHICH有指代，而B对前面两个名词都可以修饰？希望大牛帮我看看，感谢了！

B: and明显作为并列连词,与‘was completed’并列的只可能是'begun'两个谓语一个主动语态一个被动语态很显然不对，且‘begun’不对

C的错误：同前部分

我觉得'began in A.D.69'在句中过去分词做状语.不知道这样理解对不对？