12.
Travelling to conferences, lectures, workshops, and the like frequently by plane ________ as important for scientists to get together and exchange information.
A . was viewedB . is viewedC . has viewedD . is viewing
20.
(2023高一下·深圳月考)
The Xi'an City Wall is the most complete city wall that has survived China's long history. It ________ originally to protect the city in the Tang Dynasty and has now been completely restored.
A . was builtB . has been builtC . has builtD . built