In which of the following situations is the object accelerating $?$ Choose all that apply.

A motorcycle rounds a curve without changing speed.

A truck starts moving after waiting for a stoplight, traveling in a straight line.

A car drives at a constant $60mph$ as it goes over the top of a hill.

A helicopter is ascending straight upward without speeding up or slowing down.

A car slows down while driving north along a level road.

None of the objects is accelerating.

A bus drives at a constant $30mph$ as it goes down a hill with a constant incline.

A person is walking straight east at a constant speed.