下列四种现象中，哪项与生物技术无关                 （ ）

A．试管婴儿

B．克隆羊

C．转基因食品

D．水结成冰

At first he refused to admit his guilt but when he was shown the evidence he ______ and confessed (坦白).

A．broke down

B．lay down

C．got down

D．put down