Strength and content of a substance is a key factor in the entry of the substance into the body and its effects on the body.

Answer: A — True

This statement is true because the strength and content of a substance determines how much of it can enter the body. Substances with higher strengths or higher concentrations are more likely to have a greater impact on the body compared to those with lower strengths or concentrations. Option B is incorrect because the statement is true and not false. Option C is incorrect because the strength and content of a substance is mentioned in the statement as being a key factor. Option D is incorrect because the statement does not mention the variation of strength and content among substances. It simply states that it is a key factor in their entry and effects on the body.