In 1967, in response to widespread public concern aroused by medical reports of asbestos(石棉绒) related deaths, NMRC organized a committee of enquiry to investigate the health threats ,associated with the use of asbestos in the building industry. After examining evidences provided by medical researchers and building workers and management, NMRC published a report, which included advices for dealing with asbestos. The report confirmed the findings of similar research in the U. S. and Canada. Exposure to relatively small quantities of asbestos fibers, they concluded, was directly responsible for the development of cancers, asbestosis and related diseases. Taking into account evidence provided by economists and building industry management, however, the report assumed that despite the availability of other materials, asbestos would continue to play a major role in the British building industry for many years to come because of its availability and low cost. As a result, NMRC gave a series of recommendations, which were intended to reduce the risks to those who might be exposed to asbestos in working environments. They recommended that, where possible, asbestos-free materials should be employed. In cases where asbestos was employed, it was recommended that it should be used in such a way that loose fibers were less likely to enter the air. The report recommended that special care should be taken during work in environments which contain asbestos. Workers should wear protective equipment and take special care to remove dust from the environment and clothing with the use of vacuum cleaner. The report identified five factors which determine the level of risk involved. The state and type of asbestos is critical to determining the risk factors. In addition, dust formation was found to be limited where the asbestos was used when wet rather than dry. A critical factor takes place in risk reduction is the adequate ventilation of the working environment. When work takes place in an enclosed space, more asbestos particles circulate and it was therefore recommended that natural or machine ventilation should be used. By closely following these advices, it was claimed that exposure can be reduced to a reasonably practical minimum. According to the report, dust formation can be reduced when _________.

A. asbestos is damp
B. dry asbestos is employed
C. workers wear protective clothing
D. workers use asbestos in the open air