GMAT Data Sufficiency (DS) | Re: Z=2a×5b×7c A positive integer Z can be expressed in terms of its prime

Bunuel, 40 = 2^3*5 and 50 = 2*5^2.
I agree that a is atleast 3.. but how is b EXACTLY 1.. I can do Z= 200, which is 2^3*5^2*7^0 .. so b can be 2 as well right..
am I missing something?

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