It's easy to catch the flu.Just breathe.Tiny，germ－laden particles(微粒) in the air may cause half or more of all cases of the respiratory(呼吸的) infection.And that is a surprise，scientists now report.Until now，researchers thought large particles—the type that can't stay airborne long after being expelled(驱逐) by a cough or sneeze—spread most cases of influenza.
Tiny particles that can float in the air for hours to days are known as aerosols.Researchers knew that germy aerosols might spread flu.They just thought such tiny particles almost never did.Recognizing this mistake is important.Public health officials can't easily prevent infections if they don't know how most people pick up a disease.
Benjamin Cowling at the University of Hong Kong and his coworkers studied the spread of flu in 782 families.All lived in Bangkok，Thailand，or in Hong Kong and had at least one family member who became sick between 2008 and 2011.Before these illnesses，some of those families had received soap and directions on how to keep their hands clean.Some also had received masks.These masks prevent large airborne particles from getting into the wearer's nose or mouth.They may also prevent the spread of drops that spray from a wearer's mouth or nose during a cough or sneeze.But the masks don't block the far tinier aerosols.And some of those can host flu viruses.Over time，Cowling's group checked which of the volunteers they were monitoring came down with the flu.Many had been wearing masks and washing hands.
In Hong Kong，aerosols appear to have caused at least 33 in every 100 cases of flu—and maybe as many as 92 in every 100 cases.In Bangkok，aerosols caused between 55 and 98 of every 100 cases.That's a wide range of numbers，says Donald Milton at the University of Maryland in College Park.As an environmental scientist，he did not work on the new study.But，he says，what it really shows is that for flu's spread，“you can't rule out aerosols”．In fact，they can continue to cause infections long after a sick person was coughing or simply present in a room.
39．It is implied in Paragraph 1 that scientists ________．
A．were surprised that large particles spread flu easily.
B．didn't think germy aerosols could spread flu easily
C．have known tiny germ－laden particles well for many years
D．still don't know what usually results in respiratory infections
40．Paragraph 2 shows that the new discovery is very ________．
A．interesting B．surprising
C．significant D．frightening
41．What was the function of the masks in the study?
A．To attract flu viruses.
B．To prevent tiny aerosols.
C．To keep their faces clean.
D．To keep out large particles.
42．What's the writer's purpose in writing the passage?
A．To tell us how to avoid flu.
B．To advise us to keep fit.
C．To explain why flu can be spread.
D．To tell us the result of a study on flu.