原文:
Another adaptation reducing the weight of birds is the absence of some organs. Females, for instance, have only one ovary. Also, modern birds are toothless, an adaptation that trims the weight of the head.
同义替换:
Modern birds do not have teeth, an adaptation that reduces the weight of the head.
请教老师,在这里为什么用 teeth而非tooth,tooth不可用于否定句吗?