英国18世纪人口死亡率明显下降，但1816年以后死亡率上升。1831～1841年，工厂集中的伯明翰每千人死亡率由14．6上升到27．2，利物浦由21上升到34．8。导致上述情况发生的重要原因是（    ）

Welcome to Hotel Fun 4 Kids! Do you know what hotels are friendly to children or even your pets? We rate(评估) the hotels for you. You'll decide on the right hotel if you know the meanings of these symbols when you travel. You can find more information on our website(网页) on www.hotelfun4kids. com.