The study of the end－Permian mass extinction is hindered by all of the following EXCEPT __

The study of the end-Permian mass extinction is hindered by all of the following EXCEPT ______.

A．the lack of experienced researchers

B．lack of access to southern China

C．lack of relevant rock samples

D．lack of well-preserved fossils

请帮忙给出正确答案和分析，谢谢！