Which of the following statements correctly describes the position of the intake and exhaust valves during most of the power stage in a four-cycle gas engine?
a. both the intake valve and the exhaust valve are open.
b. the intake valve is open and the exhaust valve is closed.
c. the intake valve is closed and the exhaust valve is open.
d. both the intake valve and the exhaust valve are closed.

Both valves are closed during the power stroke.

While the fuel is burning in the cylinder, you want
all the force of the expanding gases to push the
piston down ... you don't want any of the gases
or their pressure escaping. 

If either of the valves was open, even just a crack,
then part of the gases would go blooey out the valve,
and some pressure would be lost that's supposed to be
pushing the piston.

The upward force acting on the wing of a airplane in flight is known as the "lift" force. (d)