He has made another wonderful discovery, _____ I think is of great importance to science.  [ ]

A. which      

B. what      

C. when    

D. that

从事特种作业（如电工、焊工、辐射、病原微生物等）的人员，必须接受相关的专业培训，通过考核并持有相应的资质证书才能上岗。