题目内容：

    As the COVID-19 pandemic （新冠肺炎）rages, with ongoing lockdown （封城）measures affecting millions of people around the globe, levels of air pollution are falling dramatically in many parts of the world. This trend has been highlighted by a new report published by Swiss company IQAir looking at ten key global cities that normally suffer from high levels of pollution.

The report compared measurements from ground-based monitoring stations regarding a type of pollutant known as PM2.5— tiny inhalable particles measuring 2.5 micrometers or smaller that are emitted （排放）by various sources such as vehicles and power plants. It found significant falls in nearly all the locations listed.

This is, perhaps, not surprising considering the large decreases in vehicle traffic and industrial activity that have occurred with lockdown measures imposed （实施）on these cities.

The report examined data collected over a three-week period for each city while they were under lockdown conditions. IQAir then compared these measurements to the same time period for the years between 2016 and 2019.

IQAir found that nine out of the ten cities in the report experienced significant reductions in PM2.5 compared to the same period for 2019. Those with historically higher levels of PM2.5 pollution saw the greatest drops including New Delhi （-60 percent） Seoul （-54 percent） and Wuhan （-44 percent）. Besides, seven of the cities saw reductions in PM2.5 of 25 to 60 percent compared to the same period last year.

Wuhan—where the COVID-19 pandemic was first identified—experienced the cleanest air on record for February and March. The city was placed under one of the strictest lockdowns in the world on January 24, which was only lifted after 10 weeks.

Unfortunately, these sharp improvements in air quality have coincided （同时发生）with significant economic disruption, suffering and loss of life across the globe. Furthermore, experts warn that without sustained action to control pollution once the crisis ends, levels will likely rebound, as has been witnessed following previous economic downturns.

1.The levels of air pollution in the nine cities drop dramatically probably because                 .

A.the global economy suffers downturns

B.vehicle traffic and industrial activity decrease sharply during the pandemic

C.the governments of these cities take effective measures to reduce air pollution

D.people around the world have raised their awareness of environmental protection

2.Why does the author list the numbers in paragraph 5?

A.To show the data from IQAir are true.

B.To make a comparison between the cities.

C.To show the significant reductions in levels of air pollution.

D.To prove the measures carried out in these cities are effective.

3.What does the underlined word "disruption” in the last paragraph mean?

A.destruction B.eruption

C.break D.construction

4.What is the main idea of the passage?

A.The introduction of PM2.5.

B.The COVID-19 pandemic has caused great loss across the globe.

C.IQAir measured the air pollution levels in major global cities during the pandemic.

D.Air pollution has fallen significantly in nine major global cities during the pandemic.