Answer: A. The amount, in dollars, the technician earned for each hour she worked at her regular job

Explanation:1. Given Information: In one week, a technician earned a total of $$900$. The equation representing the situation is $21h + 10c = 900$. $h$ = number of hours worked at her regular job. $c$ = number of hours worked at her second job. We need to find the best interpretation of $21$ in this context.

2. Analyze the Structure of the Equation: The equation $21h + 10c = 900$ shows that the total earnings ($$900$) are the sum of earnings from the regular job ($21h$) and earnings from the second job ($10c$).

3. Interpret the Term $21h$: The term $21h$ represents the total earnings from the regular job. This term is the product of the number $21$ and the variable $h$ (number of hours worked at the regular job). For the product $21h$ to result in an amount in dollars, and given that $h$ is in hours, the coefficient $21$ must represent the rate of pay in dollars per hour for the regular job.

4. Interpret the Term $10c$ (for completeness): Similarly, the term $10c$ represents the total earnings from the second job. Since $c$ is the number of hours worked at the second job, the coefficient $10$ must represent the rate of pay in dollars per hour for the second job.

5. Compare with the Given Options: [A] The amount, in dollars, the technician earned for each hour she worked at her regular job. - This matches our interpretation of $21$. [B] The amount, in dollars, the technician earned for each hour she worked at her second job. - This would be represented by $10$. [C] The number of hours the technician worked in one week at her regular job. - This is represented by $h$. [D] The number of hours the technician worked in one week at her second job. - This is represented by $c$.

6. Conclusion: The best interpretation of $21$ in the context of the equation $21h + 10c = 900$ is the amount, in dollars, the technician earned for each hour she worked at her regular job.