Similar experiments________in some European countries by the time we began to work on the project in the 1990s.
A：have been carried out
B：is being carried out
C：had been carried out
D：was being carried out
在化学反应MnO2+4HCl(浓)=MnCl2+2H2O+Cl2中，同种元素的化合价在化学反应前后变化最大的是（ ）
20世纪80年代初，联合国世界环境和发展委员会提出了____ 战略．
