Classify the carbohydrate tagatose by both the carbonyl group and the number of carbon atoms. D-tagatose is sugar with six carbon atoms. The Fischer projection shows C1 is achiral with one OH group, C2 is the location of the carbonyl, C3 and C4 have OH groups on the left, C5 has an OH groups on the right, and C6 is achiral. Enter the classification (e.g., aldotriose).