GWD-21-34 求教+讨论

完全同意楼上的，选c.

我还是比较倾向于A,

我是觉得at which有问题 如果是at which，前面是个时间点比较好吧，比如来个AD XXXXXXXX啊，BCXXXX啊

1. when it is known that humans made
2. at which it is known that humans had made
3. at which humans are known to have made
4. that humans are known to be making
5. of humans who were known to make

it is known that .....句型,科学事实用一般现在时