Why state of body and body at rest and body moving with constant velocity are equal ?

A stone of mass $0.25\; kg$ tied to the end of a string is whirled round in a circle of radius $1.5 \;m$ with a speed of $40\; rev./min$ in a hortzontal plane.the speed of the stone is increased beyond the maximum permissible value, and the string breaks suddenly, which of the following correctly describes the trajectory of the stone after the string breaks :

$(a)$ the stone moves radially outwards,

$(b)$ the stone flies off tangentially from the instant the string breaks,

$(c)$ the stone flies off at an angle with the tangent whose magnitude depends on the speed of the particle ?

We are standing in a stationary bus. When bus suddenly start why we are thrown in backward direction ?