A Block Is at Rest on a Frictionless Horizontal Plane $\theta$

Multiple Choice

A block is at rest on a frictionless horizontal plane. As one end of the plane is slowly raised, the block eventually "breaks loose" when the plane makes an angle $\theta$ , with the horizontal. The coefficient of kinetic friction can be calculated by assuming

A) constant displacement of the block.
B) constant speed of the block.
C) constant acceleration of the block.
D) none of the above.