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 inthe balanced equation. At a certain time, the mixture contains 1.8 moles of gases while the ratioof 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