EXPLANATION: Remember that a function is like a machine. F(x) is a machine and G(x) is also a machine. Plugging F into G is basically taking the output of the F machine and putting it into the G machine.
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Since diameter is double radius you can half that to get radius B is the smallest since it's radius is 3. then we have A because it is 4 then D since its double B that gives us 6 then finally you have C.