When a consumer is purchasing the best combination of two goods, X and Y, subject to a budget constraint, we say that the consumer is at an optimal choice point. A graph of an optimal choice point shows that it occurs
选项：

A:along the highest attainable indifference curve.;
B:where the indifference curve is tangent to the budget constraint.;
C:where the marginal utility per dollar spent is the same for both X and Y.;
D:All the answers are correct.