simple SAT question. confused.

Question

My friend selected option B, I did C. We're confused. Can someone please explain this for my friend?

Answer 1

No doubt Option $\,\,\,\,C\,\,\,\,\,$

Note that $a^{m+n}=a^m \cdot a^n$

Observe that \begin{align} (2x)^{3y}-(2x)^{y} \\= (2x)^{y+2y}-(2x)^{y}\\=(2x)^{y}[(2x)^{2y}-1] \end{align}

Answer 2

Option C is correct obviously because we can see that \begin{align} (2x)^{3y}-(2x)^{y} \\= (2x)^{y+2y}-(2x)^{y}\\=(2x)^{y}[(2x)^{2y}-1] \end{align} Option B is wrong because if we take out $\ 2^y $ common then we will get $\ 2^y{[2^{2y} . x^{3y}-x^y}]$