Housed in Milan, one of Europe's dirtiest cities, Leonardo da Vinci's masterpiece The Last Supper still faces a threat from air pollution, researchers say, despite preventative measures that have significantly decreased pollutants in the church where the famous painting is on display.
In 2009, Italian authorities installed a heating, ventilation (通风) and airconditioning system to protect t
he painting from Milan's polluted air. To see whether the system could improve air quality, a team of researchers led by Constantinos Sioutas, a professor of civil and environmental engineering at the University of 
Southern California, placed two airqu
ality monitors inside the church for a year to collect samples from around painting. Results showed the levels of fine and coarse particulates were reduced near the painting by 88 percent and 94 percent, respectively, compared with the corresponding outdoor levels. “It's a spectacular reduction，” Sioutas said in a statement. “It is, frankly, very impressive.”
Although the researchers applauded the successful decrease in particulates around da Vinci's painting, they warned that indoor sources of pollution may still pose a threat to the priceless painting. Fatty lipids (油脂) from the skin of customers in the church appeared in significant quantities around the painting, despite the fact that visitors getting access to the artwork are strictly regulated.
Only a few visitors are allowed inside the church at a time, and they enter the building and can stay for only 15 minutes each. However, fatty lipids from visitors' skin can combine with dust in the air and pollute the painting if they come in contact with it, says study researcher Nancy Daher of the University of Southern California.
Even previous restorations of the painting may pollute its surface, Daher said. Tiny particulates of the wax applied during early repair efforts can be emitted from the painting, get into the air and tarnish (使失去光泽) the painting in the same manner as the fatty lipids.
The researchers noted that their airsample analysis could be used as a point of reference for future studies aimed at protecting valuable artwork and artefacts.
1．What does the underlined word “particulates” in Paragraph 2 probably mean?
A．The smallest parts of a chemical substance.
B．Things dirtier than usual.
C．Certain types of gas.
D．Extremely small pieces of dirt, which cause serious pollution.
2．According to the text, the following factors pollute the painting EXCEPT ________．
A．fatty lipids from the skin
B．tiny particles of the wax
C．breath of the visitors
D．dust in the air
3．According to the researchers, which of the following is NOT true of their airsample analysis?
A. The airsample analysis was got by gathering air in the church for a year.
B．The airsample analysis showed that the painting was completely away from air pollution.
C．The airsample analysis can be helpful for future studies protecting valuable artwork.
D．The airsample analysis showed that the particulates around da Vinci's painting were decreased successfully.
4．What can be the best title of the text?
A．How to decrease levels of particulates effectively
B．The famous painting —da Vinci's The Last Supper
C．Keeping air pollution from damaging da Vinci's The Last Suppers
D．How to protect artwork in households