单项选择题

MEMO
From: Dr Ohao Liang Luen, Chief Medical Officer
To: All Managers of the Dragon Company’s Branch Factories
Date: 29 November
Loss of hearing among workers in our factories is now becoming a very serious problem of which all managers and supervisors must be aware. It is important to identify workers whose hearing may have (19) because of unacceptable noise in certain (20) of a factory. Where there is very noisy machinery, supervisors usually (21) ear protectors to operatives and to those working nearby. However, not all workers are willing to wear ear protectors and often (22) instructions to do so. Others put them on but then (23) them unless they are (24) supervised. Appropriate, training and information about the (25) which can be caused by noise is probably the most effective way of dealing with this problem, provided that it is followed up by (26) vigilance.
Sometimes the ear protectors worn by workers may be inadequate but may not cause any (27) loss of hearing. In such cases hearing difficulties will (28) so gradually that those affected may hardly be aware of them. In most cases, this kind of gradual deafness is (29) only when it is too late to (30) the situation in any way.
Harder to recognize are noise (31) which do not cause deafness but which nevertheless (32) employees. It is therefore just as important that workers’ complaints about such noise are fully (33) .

31（）

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