21. The answer is $\mathbf{( E )}$.

From the fact that Agilulfo was not punished, we deduce that: yesterday Agilulfo did not get a bad grade, or yesterday afternoon the mother was not at home. On the other hand, we cannot say with certainty that one of these two circumstances actually occurred, nor, for that matter, can we assert that one of them is false. Therefore, we cannot be certain that any of the statements (A), (B), (C), (D) is true.
21) Every time Agilulfo comes home from school after getting a bad grade, if his mom is at home, she puts him in time-out. Knowing that yesterday afternoon Agilulfo was not put in time-out, which of the following statements is certainly true? (A) yesterday Agilulfo got a bad grade, (B) yesterday Agilulfo did not get a bad grade, (C) yesterday afternoon his mom was at home, (D) yesterday afternoon his mom was not at home, (E) none of the previous statements is certainly true.