Question

the noise level or volume can be measured in decibels: db(a). some examples: normal conversation: 60 db(a) electrical appliances: > 85 db(a) power drill: 100 db(a) it is advisable to use ear protection at noise levels greater than 80 db(a). common types of ear protection: ear muffs are worn over the ears. custom molded ear plugs individually made - to - measure ear protection. ear plugs the ear plug is kneaded and then put in the ear. on expansion, the hearing channel is closed off, resulting in noise reduction. note: dont ever use combinations of ear protection. the noise via bone conduction can damage hearing. what should you bear in mind when using ear protection? ear protection is often advised, but never mandatory. ear protection has an unlimited service life. never wear combinations of ear protection.

Explanation:

The text clearly states "Don't ever use combinations of ear protection. The noise via bone conduction can damage hearing." So we must avoid wearing combinations of ear - protection. There is no mention of ear protection being mandatory or having an unlimited service life in the relevant text about things to bear in mind.

Answer:

Never wear combinations of ear protection.