答案：B

解析：

Choice B is correct. The price per ounce of a material multiplied by the amount of that material in ounces is the total amount, in dollars, spent on that material.

Therefore, the total amount spent on copper plus the total amount spent on tin must be $90. The cost, in dollars, of 1 ounce of tin can be interpreted by looking at the term 1.08t in the given equation. This term represents the total amount spent on tin. Thus, the price of one ounce of tin is 1.08 dollars.