等比数列$\left\{a_n\right\}$中，$a_1=2,a_8=4$，函数$f\left(x\right)=x(x-a_1)(x-a_2)......(x-a_n)$，则$f`\left(0\right)=$

$2^6$ $2^8$ $2^{12}$ $2^{15}$