blackening of teeth

例句：

1. 1. The dentist advised him to brush his teeth twice a day.
   牙医建议他每天刷牙两次。
2. 2. She had her teeth blackened for cosmetic reasons.
   她为了美容原因把牙齿黑化了。
3. 3.The dark-skinned man's teeth were noticeably blacker than the fair-skinned woman's.
   这个黑人的牙齿比那个皮肤白皙的女人的明显要黑。