Double more than; more than double

Hello~

This test question asks people to choose one phrase which fits the semantics.

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Temping can help you learn new skills or bridge the gap between jobs. Some people are even making a career of temping—mostly those who like the independence or need more flexible work schedules, such as working mothers. All told, companies shelled out $5 billion to temps in 1995, ____ what they paid four years earlier, according to the National Association of Temporary and Staffing Services. [...]

a) double more than
b) more than double
c) more double than
d) double than more

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The given answer is (b)
I myself ruled (c) and (d) out.
But what are the differences in meaning with (a) and (b)?

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All told, companies shelled out $5 billion to temps in 1995, more than double what they paid four years earlier, ...
With (b) filled the blank, is double a verb? Does more than mean very much?

I took a look at this page.

However, reading this page failed to make me clear, because I cannot find its subject with (b) filled in.

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Or is there a hidden which is?

All told, companies shelled out $5 billion to temps in 1995, which is more than double what they paid four years earlier, ...

which refers to 5 billion?
double is a noun?

Regards