A cart from a supermarket in my neighborhood was rolling along on its own, with no one pushing it. It was a cart just like all the others, made of thick wire, with four little rubber wheels (the front pair slightly closer together, which is what gives the vehicle its characteristic shape), and a bar coated with bright red plastic for steering it around. There was nothing to distinguish it from the two hundred other carts that belonged to that enormous supermarket, the biggest and busiest in the neighborhood. Except that the cart I’m referring to was the only one that moved on its own. It did this with infinite discretion: in the tumult that reigned on the premises from opening to closing time, to say nothing of the peak hours, its movement went unnoticed.

Which answer choice explains what is left uncertain in this passage?

(1 point)
Responses

what the cart looks like
what the cart looks like

why the cart is able to move on its own
why the cart is able to move on its own

where the cart is
where the cart is

what type of cart is being described

what the cause of the cart moving on its own is