麻烦老师帮忙解释一下这个问题

问题如下：

According to the Stockholm Resilience Centre, how many of the nine planetary boundaries have already been crossed as a result of human activity?

选项：

A.Three B.Six C.All nine

解释：

B is correct. According to an update by the Stockholm Resilience Centre from 2022, six of nine planetary boundaries have already been crossed as a result of human activity: climate change; loss of biosphere integrity; land-system change; freshwater (green water boundary); novel entities (including plastic pollution); and altered biogeochemical cycles (phosphorus and nitrogen loading).