It was also partly motivated by an attempt to understand the extent to which antiretroviral treatment is reducing overall levels of mortality in South Africa.求大神分析这个句子,尤其是to which的作用,

It was also partly motivated by an attempt to understand the extent to which antiretroviral treatment is reducing overall levels of mortality in South Africa.求大神分析这个句子,尤其是to which的作用,
It was also partly motivated by an attempt to understand the extent to which antiretroviral treatment is reducing overall levels of mortality in South Africa.求大神分析这个句子,尤其是to which的作用,

It was also partly motivated by an attempt to understand the extent to which antiretroviral treatment is reducing overall levels of mortality in South Africa.求大神分析这个句子,尤其是to which的作用,
to some/the extent 某种程度,wich引导定语从句,先行词为extent,介词保留.

which 可以省略，to起介词的作用。