Why do scientists need to know the LD50 of a substance?

The LD50 gives a measure of the immediate or acute toxicity of a chemical in the strain, sex, and age group of a particular animal species being tested. ... It is also important to know that the actual LD50 value may be different for a given chemical depending on the route of exposure (e.g., oral, dermal, inhalation).

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the answer is third world countries, take for instance china, they restricted each family with only one child,

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the answer is b.) hydrogen bonds between bases.