单选题 It has been recognized that the causes of premature births are multifarious, and it has been speculated that environmental______, such as stress, fear, exile, and inadequate prenatal maternal care, contribute to the rate of premature births.
【正确答案】 C
【答案解析】解析：句意：现在已经意识到早产的原因是多种多样的，而且据推测环境因素，例如压力，恐惧，流放，不充分的产前孕妇保健等是导致早产率上升的原因。element、ingredient、factor均有“组成成分”的意思：element指元素或某特定类型的人或群体，分子。ingredient指原料、成分。factor指(产生特定结果或状态的)因素，要素，侧重指原因。momentum意为“势头；动力”。结合语境，factor为最佳答案，因此选择C项。