单选题：根据材料请回答Drug Reactions:a Major Cause of DeathAdverse drug rea

根据材料请回答Drug Reactions:a Major Cause of Death
Adverse drug reactions may cause the deaths of over 100,000 US hospital patients each year, making them a leading cause of death nationwide, according to a report in The Journal of the American Medical Association .
"The incidence of serious and fatal adverse drug reactions (ADRs) in US hospital was found to be extremely high,” say researchers at the .University of Toronto in Ontario,Canada.
They carried on an analysis of 39 ADR-related studies at US hospitals over the past 30 years and defined an ADR as "any harmful, unintended, and undesired effect of a drug which occurs at doses used in humans for prevention, diagnosis, or therapy."An average 6.7% of all hospitalized patients experience an ADR every year, according to the research-ers.They estimate that "in 1994, overall 2,216,000 hospitalized patients had serious ADRs, and 106,000 had fatal ADRs." This means that ADRs may rank as the fourth sin-gle largest cause of death in America.
And these incidence figures are probably conservative, the researchers add, since their ADR definition did not include outcomes linked to problems in drug administration, over-doses, drug abuse, and therapeutic failures.
The control of ADRs also means spending more money.One US study estimated the overall cost of treating ADRs at up to $4 billion per year.Dr David Bates of Brigham and Women's Hospital in Boston, Massachusetts, believes that healthcare workers need to pay more attention to the problem, especially since many ADRs are easily preventable." When a patient develops an allergy or sensitivity, it is often not recorded," Bates notes," and patients receive drug to which they have known allergies or sensitivities with distur-bing frequency." He believes computerized surveillance systems still work-in-progress at many of the nation's hospitals should help cut down the frequency of these types of er-rors.Researchers at the University of Toronto believe that________. A．ADRs have caused medical problems, though they seldom lead to death
B．ADRs have very often caused patients to die in Canada
C．ADRs have caused many deaths in America over the past 30 years
D．It is easy to prevent ADRs from happening