One way to solve this problem is to find the actual areas of the 8 inch diameter pie, and the area of the 12 inch diameter pie, and divide the latter by the former. But this problem can be solved almost instantly if you know a very useful rule: if you multiply any length of one shape to get the length of a similar shape, then if you want the area of the latter shape, you can simply square the multiplier you just used. Here's a simple example. If you have two similar triangles, and the perimeter of the larger triangle is 3 times that of the smaller triangle, then you know the area is 3^2 times larger, or 9 times larger.
So, using this technique, when we see that the ratio of pie diameter is 12 to 8, or 3/2, we can simply square 3/2, getting 9/4, which is B.
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