71. One mole of X(g) and one mole of Y(g) are mixed in a closed reactor in the presence of catalysts, and Z(g) is generated. The reaction is a X + b Y → c Z, where a, b, and c are the coefficients in the balanced equation. At a certain time, the mixture contains 1.8 moles of gases while the ratio of their partial pressures is PX:PY:PZ = 7:9:2. What are the values of a, b, and c? (A) a = 1, b = 2, c = 3 (B) a = 3, b = 1, c = 2 (C) a = 7, b = 9, c = 2 (D) a = 3, b = 1, c = 8 (E) a = 2, b = 9, c = 7