Which one of the following statements is correct about the term $($TLV$)?$

a$.$ A concentration level in ppm $($part of the vapor per million part of contaminated air$)$ up to which a worker can be exposed for $12$ hours without any adverse health effect

b$.$ A concentration level expressed in ppm $($part of the vapor per million part of contaminated air$)$ up to which it is believed a worker can be exposed day after day for working without any adverse health effect

c$.$ A concentration level in ppm $($part of the vapor per million part of contaminated air$)$ to which it is believed a worker can be exposed for $4$ hours without any adverse health effect

d$.$ A concentration level in ppm $($part of the vapor per million part of contaminated air$)$ up to which a worker can be exposed for $24$ hrs$.$ without any adverse health effects

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