Ruining the Ruins Acid rain (酸雨) is now a familiar problem in the industrialized countries in Europe.Harmful gases are produced by power stations and cars. They dissolve in rainwater and this makes acid rain, which damages trees, rivers and streams. Acid rain is also capable of dissolving some rocks, and buildings made of soft rock,such as limestone(石灰石),are particularly badly affecteD．The acid rain attacks the rock,and so carvings and statues are eroded(受腐蚀)more quickly. __________(46) .According to a report in the New Scientist, acid rain is being blamed for the rapid decay of ancient ruins in Mexico. The old limestone buildings in places like Chichen Itza, Tulum and Palenque are wearing away very quickly indeed.These sites are the remains of the buildings built by the Mayas between 250 BC and AD 900,and the spectacular ruins of Mayan civilization are visited by thousands of tourists every year. But those ruins are in danger of being seriously damaged by pollution. At many sites the stone has been covered with a layer of black substance. __________ (47) Scientists estimate that about one millimeter of stone is worn away every twelve years. __________ (48) The acid rain is said to be caused by pollution from oil wells in the Gulf of Mexico. Car exhaust Oases are also a problem. Local volcanic eruptions make the problem even worse. Nevertheless, with enough money and effort, researchers say that many of the problems could be solved and the rate of erosion reduceD．__________ (49). Mexico's current lack of funds is also partly due to oil. The country has rich oil fields and a few years ago, when oil was expensive, Mexico was selling large quantities of oil to the USA and earning a lot of money. __________ (50) However, the price of oil then dropped, and Mexico has been left owing enormous sums of money and with not enough income from oil sales to pay back the loans. So unless the price of oil rises, it is unlikely that Mexico will be able to afford to clean up the pollution and save its Mayan ruins from destruction.