Use the Derivative Of $f ( x ) = 5 x ^ { 3 } + 15 x$

Multiple Choice

Use the derivative of $f ( x ) = 5 x ^ { 3 } + 15 x$ to determine any points on the graph of $f ( x )$ at which the tangent line is horizontal.

A) $( 1,20 )$
B) $( 1,20 )$ and $( - 1 , - 20 )$
C) $( 5,700 )$ and $( - 5,700 )$
D) $( 0,0 )$
E) $f ( x )$ has no points with a horizontal tangent line.