Give this to _____ you think can do the work well.     

A who          B whoever          C whomever     D however      

引起原发性肝癌最常见的病因是

A．乙型、丙型肝炎病毒感染

B．食物中致癌物质

C．亚硝胺类化学物质

D．寄生虫感染

E．细菌感染