12. What is the oxidation state of chromium in $\text{K}_2\text{Cr}_2\text{O}_7$? (A) 4 (B) -2 (C) 3 (D) 6

19. Which of the following molecules (or ions) are nonlinear? $$\text{NO}_2^-,\quad\text{C}_2\text{H}_2,\quad\text{N}_3^-,\quad\text{HCN},\quad\text{CO}_2,\quad\text{H}_2\text{O}_2$$ (A) $\text{C}_2\text{H}_2,\ \text{HCN}$ (B) $\text{CO}_2,\ \text{N}_3^-$ (C) $\text{NO}_2^-,\ \text{H}_2\text{O}_2$ (D) $\text{N}_3^-,\ \text{NO}_2^-$ (E) all are linear