Pigeons are often seen as dirty birds and an urban nuisances. 未知类型：{'label': 'questionDesc', 'content': '请指出以下句子中的错误。', 'isMemberControl': 0, 'type': 181} 未知类型：{'label': 'source', 'content': '2017年12月 四级 卷一 选词填空', 'isMemberControl': 0, 'type': 181} A: dirty birds B: urban nuisances C: are often seen

What causes the chemicals to build up and harm us in offices? As _______ and homes often have inadequate ventilation, these chemicals can ______ to become health nuisances. Their toxic _______ are only now being slowly _______.