There is a system ________the waste is disposed of using the principles of ecology.

A．which

B．where

C．that

D．at which

哺乳仔猪，表现呆滞，体温升高，口腔黏膜有小水泡。剖检见心脏稍扩张、散在分布灰黄色和条纹状病灶，沿心冠部横切有灰黄色条纹围绕心脏并呈环层状排列。该病的心脏病变为（）

A、虎斑心

B、绒毛心

C、盔甲心

D、桑葚心

E、菜花心