Assertion (A) is true because the context states that water always contains hydrogen and oxygen in a fixed mass ratio of 1:8. Therefore, 2 g of hydrogen and 16 g of oxygen (which maintains the 1:8 ratio) will combine to form 18 g of water. Reason (R) is false because Dalton's Atomic Theory explains that atoms combine in simple whole number ratios of atoms, not by mass. The fixed ratio by mass is described by the Law of Constant Proportions, not Dalton's postulate. Thus, A is true but R is false.