疾病的外因
参考答案：
生物性致病因素；化学性致病因素；物理性致病因素；营养性致病因素。
物流活动只能在一个很小的区域内进行。( )
In spoken English of some areas in the US, the "r" sounds at the end of the words（）
A. are dropped
B. drop
C. are being dropped
D. have dropped
