Two puppies, Walter and Stanley, are growing at different but constant rates. Walter's mass is $12 \mathrm{~kg}$ and he is growing at a rate of $2 \mathrm{~kg} /$ month. Stanley's mass is $6 \mathrm{~kg}$ and he is growing at a rate of $2.5 \mathrm{~kg} / \mathrm{month}$. What will Stanley's mass be when it is equal to Walter's?
(A) $24 \mathrm{~kg}$
(B) $28 \mathrm{~kg}$
(C) $32 \mathrm{~kg}$
(D) $36 \mathrm{~kg}$
(E) $42 \mathrm{~kg}$
In the table below, we list the mass of each dog at the end of each month.

| Month | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: |
| Walter's mass (in kg) | 12 | 14 | 16 | 18 | 20 | 22 | 24 | 26 | 28 | 30 | 32 | 34 | 36 |
| Stanley's mass (in kg) | 6 | 8.5 | 11 | 13.5 | 16 | 18.5 | 21 | 23.5 | 26 | 28.5 | 31 | 33.5 | 36 |

After 12 months have passed, Stanley's mass is $36 \mathrm{~kg}$ and is equal to Walter's mass.

(Note that since Stanley's mass is increasing at a greater rate than Walter's each month, this is the only time that the two dogs will have the same mass.)

ANSwER: (D)