If $C(x)=15x^3-3\sqrt{5}x+\sqrt{7}$ represents the total cost of producing x items, the slope of the marginal cost function at x =1 is :

$45-3\sqrt{5}$ $90+3\sqrt{5}$ $\frac{15x^4}{4}-\frac{3\sqrt{5}}{2}+\sqrt{7}x+C$ 90

90