In a chemical experiment, 25 g of reactant A reacts completely with 35 g of reactant B to form 50 g of product C and a certain amount of product D. Based on the law of conservation of mass, what is the mass of product D?

Country A exports 100 units of good X at a price of USD 10 per unit and imports 80 units of good Y at USD 12.50 per unit. If in the following year, the export price rises to USD 12 and the import price rises to USD 15, what is the Commodity Terms of Trade index in Year 2 (base Year 1 = 100)?

The labor cost incurred in producing 1 unit of Good A and Good B in India is ₹20 and ₹40 respectively, whereas in China it is ₹40 and ₹20 respectively. What are the opportunity costs of producing Good B in India and China respectively?