20 GWD17-Q20:

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,[h1] 

2. officially known as the Flavian Amphitheater, begun in A.D. 69, during the reign of Vespasian, and[h2] 

3. which was officially known as the Flavian Amphitheater, began in A.D. 69, during the reign of Vespasian, and[h3] 

4. officially known as the Flavian Amphitheater and begun in A.D. 69, during the reign of Vespasian it[h4] 

5. officially known as the Flavian Amphitheater, which was begun in A.D. 69, during the reign of Vespasian, and[h5] 

弄错，应该选c,词典里有主动语态表被动，

begin 表示“开始”，其内涵是“使处于进程中”
Work on it had begun before my sister left.
这座房子在我姐姐离开之前就已经动工了。

a play that begins at eight o'clock;