The TV play is about a true story _____happened in Mianyang in 1998.[ ]

A. it                  

B. what

C. that                

D. when

1976年Wolfe等根据乳腺实质和脂肪所占比例与发生乳腺癌的危险性关系将乳腺分为___、___、___和___4型。