卫生官员将食物中毒的爆发追溯到了一些受污染的贝类。

Health officials ___traced the outbreak of food poisoning to some contaminated shellfish.

复活节赠送彩蛋的传统可以追溯到中国古代的节日。

The tradition of giving eggs at Easter can be ___traced back to festivals in ancient China.

调查人员至今仍未能追踪到那个有害谣言的源头。

Investigators have not yet been able to ___trace the source of the damaging rumour.