从A、B、C、D四个选项中，选出可以填入空白处的最佳选项。
21. There is ____ universal concept among ____ parents that the more they feed their kids, the healthier they will grow up to be.
A. an, the B. a, the C. a, / D. /, the
22. My brother is really _____. He often works in his office far into the night.
A. open-minded B. hard-working C. self-confident D. warm-hearted
