If you ignore friction, the amount of time required for a box to slide down an inclined plane is square root of 2a/g sin theta cos theta
where a is the horizontal distance defined by the inclined plane, g is the acceleration due to gravity and

theta

is the angle of the inclined plane. For what values of

theta

is the expression undefined?

a. 0°

b. 90°

c. both a and b

d. none