【单选题】Suppose a consumer has preferences over two goods, X and Y, which are perfect substitutes. In particular, two units of X is equivalent to one unit of Y. If the price of X is $1, the price of Y is $3, ...
A.
30 units of X and 0 units of Y
B.
0 units of X and 10 units of Y
C.
15 units of X and 5 units of Y
D.
15 units of X and 0 units of Y