31. There were 5,000 reported cases of ADIS last year, _____ nearly 3,000 have already died.

B die of 固定搭配

若死因存在于人体之上或之内（主要指疾病、衰老等自身的原因)，一般用介词 of。如：die of illness (heart trouble, cancer, a fever, etc) 死于疾病 (心脏病，癌症，发烧等)

D. 定语从句
Nearly 3,000 of 5,000 reported cased have already died.5,000 reported 在定语从句中用which代掉。

B
不过我觉得也可以说是OF是用来修饰和限定5000的,意为:去年据报道有5000例艾滋病,其中死亡人数已达几3000人左右.
~~~~仅个人见解~~~~

D
其中