Final answer:
A must at 21° Brix contains 210 grams of sugar per liter, as the Brix scale indicates 1 degree Brix equals 1 gram of sucrose per 100 milliliters, and this is multiplied by 10 to convert to liters.
Step-by-step explanation:
To determine how many grams of sugar per litre contains a must at 21° Brix, we can refer to the definition of the Brix scale. The Brix scale measures the sugar content of an aqueous solution, where 1 degree Brix is equivalent to 1 gram of sucrose in 100 milliliters of solution. Therefore, a must at 21° Brix contains 21 grams of sugar per 100 milliliters.
We need to convert this to liters, so we multiply by 10 (since there are 1000 milliliters in a liter). This gives us:
21 grams/100 mL x 10 = 210 grams of sugar per liter.
So, the correct answer to the question is B. 210 grams.