"Normally, jet engines push air out the back of the engine, resulting in forward thrust, but commercial aircraft often have thrust reversers that can change the direction of the ejected air, sending it forward." "When might thrust reversers be useful in practice? (Chose all that apply)

a) If the ejected air is directed forward then thrust force is backward. This might be desirable to conserve the speed of the plane.

b) If the ejected air is directed downward, the thrust force is up. Jets can take off vertically without needing a runway this way.

c) If the ejected air is directed forward then thrust force is backward. This might be desirable to accelerate the plane.

d) If the ejected air is directed forward then thrust force is backward. This might be desirable to slow the plane down. "