Choice A
The polynomial for choice A can be written as 5y^2+6y+3. The leading term is 5y^2. The coefficient for this term is 5, so the leading coefficient is 5
So Choice A is one of the answers. It turns out that choice A is the only answer (see below)
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Choice B
The leading coefficient for x+3 is 1
This is because x+3 is the same as 1x+3
We can rule out choice B since the leading coefficient is not 5
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Choice C
Rearrange the terms in x^2-3x^3+2x to get -3x^3+x^2+2x
The leading coefficient is -3
We can rule out choice C
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Choice D
The leading coefficient here is 3 because the standard form is 3y^2+5y-4
We can rule out choice D
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Summary: The only answer is choice A
