本文为说明文，主要介绍威尼斯引以为豪的古建筑正承受着来自人类与大自然的双重威胁。
1．D 细节理解题。根据第一段and nearly everything is remarkable, from the magnificent Piazza San Marco (St. Mark’s Square) and the palaces that line the Grand Canal to the centuriesold homes of simple fishermen.可判断威尼斯以它杰出的古建筑而闻名，故选D。
2．A 细节理解题。根据第一段It would be easy to say that the city itself is an enormous museum if it were not for the fact that it is so obviously alive.可推断作者认为威尼斯是巨大的博物馆，故选A。
3．A 细节理解题。根据第三段… the wash produced by the constantly increasing number of motor boats that speed up and down the canals.可判断选A。
4．D 细节理解题。根据第四段A further threat to Venice comes from the Venetians themselves可知人为因素比自然因素对威尼斯造成的损坏更大，故选D。
